diff --git a/LayoutTests/ChangeLog b/LayoutTests/ChangeLog
index f7f9604..31868e1 100644
--- a/LayoutTests/ChangeLog
+++ b/LayoutTests/ChangeLog
@@ -1,3 +1,21 @@
+2020-02-13  Kate Cheney  <katherine_cheney@apple.com>
+
+        Expose prevalent domains on a per-page basis
+        https://bugs.webkit.org/show_bug.cgi?id=207523
+        <rdar://problem/59270758>
+
+        Reviewed by Chris Dumez.
+
+        Added 2 new tests for the resource load statistics database and
+        memory stores. Added a basic iframe that both tests can use to test
+        if a resource load from a prevalent domain gets reported properly.
+
+        * http/tests/resourceLoadStatistics/prevalent-domains-per-page-database-expected.txt: Added.
+        * http/tests/resourceLoadStatistics/prevalent-domains-per-page-database.html: Added.
+        * http/tests/resourceLoadStatistics/prevalent-domains-per-page-expected.txt: Added.
+        * http/tests/resourceLoadStatistics/prevalent-domains-per-page.html: Added.
+        * http/tests/resourceLoadStatistics/resources/basic-iframe.html: Added.
+
 2020-02-13  Lauro Moura  <lmoura@igalia.com>
 
         [GTK] Gardening more flaky crashes
diff --git a/LayoutTests/http/tests/resourceLoadStatistics/prevalent-domains-per-page-database-expected.txt b/LayoutTests/http/tests/resourceLoadStatistics/prevalent-domains-per-page-database-expected.txt
new file mode 100644
index 0000000..97e1abb
--- /dev/null
+++ b/LayoutTests/http/tests/resourceLoadStatistics/prevalent-domains-per-page-database-expected.txt
@@ -0,0 +1,10 @@
+Tests that a iframe from a prevalent domain will be reported as prevalent under this page.
+
+On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE".
+
+
+PASS Domain was successfully marked prevalent.
+PASS successfullyParsed is true
+
+TEST COMPLETE
+
diff --git a/LayoutTests/http/tests/resourceLoadStatistics/prevalent-domains-per-page-database.html b/LayoutTests/http/tests/resourceLoadStatistics/prevalent-domains-per-page-database.html
new file mode 100644
index 0000000..812eaee
--- /dev/null
+++ b/LayoutTests/http/tests/resourceLoadStatistics/prevalent-domains-per-page-database.html
@@ -0,0 +1,74 @@
+<!DOCTYPE html>
+<html>
+<head>
+    <script src="/js-test-resources/js-test.js"></script>
+    <script src="/js-test-resources/ui-helper.js"></script>
+    <script src="/resourceLoadStatistics/resources/util.js"></script>
+    <script>
+        description("Tests that a iframe from a prevalent domain will be reported as prevalent under this page.");
+        jsTestIsAsync = true;
+
+        function finishTest() {
+            setEnableFeature(false, finishJSTest);
+        }
+
+        const iframeID = "iFrameFromPrevalentDomain";
+        function askForPrevalentResources() {
+            testRunner.getPrevalentDomains(function (arrayOfDomains) {
+                var passed = true;
+                for (var i = 0; i < arrayOfDomains.length; ++i) {
+                    if (arrayOfDomains[i] === "localhost") {
+                        passed = true;
+                        break;
+                    }
+                }
+                if (passed)
+                    testPassed("Domain was successfully marked prevalent.");
+                else
+                    testFailed("Domain was not successfully marked prevalent.");
+                finishTest();
+            });
+        }
+
+        function activateElement(elementId) {
+            var element = document.getElementById(elementId);
+            var centerX = element.offsetLeft + element.offsetWidth / 2;
+            var centerY = element.offsetTop + element.offsetHeight / 2;
+            UIHelper.activateAt(centerX, centerY).then(
+                function () { },
+                function () {
+                    testFailed("Promise rejected.");
+                    finishTest();
+                }
+            );
+            askForPrevalentResources();
+        }
+
+        function runTest() {
+            if (document.location.hash !== "#elementActivated") {
+                document.location.hash = "elementActivated";
+                activateElement(iframeID);
+            }
+        }
+
+        const hostUnderTest = "localhost:8000";
+        const statisticsUrl = "http://" + hostUnderTest;
+        
+        testRunner.setUseITPDatabase(true);
+        setEnableFeature(true, function() {
+            testRunner.setStatisticsPrevalentResource(statisticsUrl, true, function() {
+                if (!testRunner.isStatisticsPrevalentResource(statisticsUrl))
+                    testFailed("Host did not get set as prevalent resource.");
+
+                let iframeElement = document.createElement("iframe");
+                iframeElement.onload = runTest;
+                iframeElement.id = "iFrameFromPrevalentDomain";
+                iframeElement.src = "http://localhost:8000/resourceLoadStatistics/resources/basic-iframe.html";
+                document.body.appendChild(iframeElement);
+            });
+        });
+    </script>
+</head>
+<body>
+</body>
+</html>
diff --git a/LayoutTests/http/tests/resourceLoadStatistics/prevalent-domains-per-page-expected.txt b/LayoutTests/http/tests/resourceLoadStatistics/prevalent-domains-per-page-expected.txt
new file mode 100644
index 0000000..97e1abb
--- /dev/null
+++ b/LayoutTests/http/tests/resourceLoadStatistics/prevalent-domains-per-page-expected.txt
@@ -0,0 +1,10 @@
+Tests that a iframe from a prevalent domain will be reported as prevalent under this page.
+
+On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE".
+
+
+PASS Domain was successfully marked prevalent.
+PASS successfullyParsed is true
+
+TEST COMPLETE
+
diff --git a/LayoutTests/http/tests/resourceLoadStatistics/prevalent-domains-per-page.html b/LayoutTests/http/tests/resourceLoadStatistics/prevalent-domains-per-page.html
new file mode 100644
index 0000000..c18c6fb
--- /dev/null
+++ b/LayoutTests/http/tests/resourceLoadStatistics/prevalent-domains-per-page.html
@@ -0,0 +1,73 @@
+<!DOCTYPE html>
+<html>
+<head>
+    <script src="/js-test-resources/js-test.js"></script>
+    <script src="/js-test-resources/ui-helper.js"></script>
+    <script src="/resourceLoadStatistics/resources/util.js"></script>
+    <script>
+        description("Tests that a iframe from a prevalent domain will be reported as prevalent under this page.");
+        jsTestIsAsync = true;
+
+        function finishTest() {
+            setEnableFeature(false, finishJSTest);
+        }
+
+        const iframeID = "iFrameFromPrevalentDomain";
+        function askForPrevalentResources() {
+            testRunner.getPrevalentDomains(function (arrayOfDomains) {
+                var passed = false;
+                for (var i = 0; i < arrayOfDomains.length; ++i) {
+                    if (arrayOfDomains[i] === "localhost") {
+                        passed = true;
+                        break;
+                    }
+                }
+                if (passed)
+                    testPassed("Domain was successfully marked prevalent.");
+                else
+                    testFailed("Domain was not successfully marked prevalent.");
+                finishTest();
+            });
+        }
+
+        function activateElement(elementId) {
+            var element = document.getElementById(elementId);
+            var centerX = element.offsetLeft + element.offsetWidth / 2;
+            var centerY = element.offsetTop + element.offsetHeight / 2;
+            UIHelper.activateAt(centerX, centerY).then(
+                function () { },
+                function () {
+                    testFailed("Promise rejected.");
+                    finishTest();
+                }
+            );
+            askForPrevalentResources();
+        }
+
+        function runTest() {
+            if (document.location.hash !== "#elementActivated") {
+                document.location.hash = "elementActivated";
+                activateElement(iframeID);
+            }
+        }
+
+        const hostUnderTest = "localhost:8000";
+        const statisticsUrl = "http://" + hostUnderTest;
+        
+        setEnableFeature(true, function() {
+            testRunner.setStatisticsPrevalentResource(statisticsUrl, true, function() {
+                if (!testRunner.isStatisticsPrevalentResource(statisticsUrl))
+                    testFailed("Host did not get set as prevalent resource.");
+
+                let iframeElement = document.createElement("iframe");
+                iframeElement.onload = runTest;
+                iframeElement.id = "iFrameFromPrevalentDomain";
+                iframeElement.src = "http://localhost:8000/storageAccess/resourceLoadStatistics/basic-iframe.html";
+                document.body.appendChild(iframeElement);
+            });
+        });
+    </script>
+</head>
+<body>
+</body>
+</html>
diff --git a/LayoutTests/http/tests/resourceLoadStatistics/resources/basic-iframe.html b/LayoutTests/http/tests/resourceLoadStatistics/resources/basic-iframe.html
new file mode 100644
index 0000000..2655f4e
--- /dev/null
+++ b/LayoutTests/http/tests/resourceLoadStatistics/resources/basic-iframe.html
@@ -0,0 +1,7 @@
+<!DOCTYPE html>
+<html>
+<head>
+</head>
+<body>
+</body>
+</html>
diff --git a/Source/WebCore/ChangeLog b/Source/WebCore/ChangeLog
index 2e0633c..4c2d446 100644
--- a/Source/WebCore/ChangeLog
+++ b/Source/WebCore/ChangeLog
@@ -1,3 +1,24 @@
+2020-02-13  Kate Cheney  <katherine_cheney@apple.com>
+
+        Expose prevalent domains on a per-page basis
+        https://bugs.webkit.org/show_bug.cgi?id=207523
+        <rdar://problem/59270758>
+
+        Reviewed by Chris Dumez.
+
+        Tests: http/tests/resourceLoadStatistics/prevalent-domains-per-page-database.html
+               http/tests/resourceLoadStatistics/prevalent-domains-per-page.html
+
+        Logs subresource loads in CachedResourceLoader::requestResource, and
+        redirects in ResourceLoader::willSendRequestInternal,
+        and reports domains to the Web Process.
+
+        * loader/FrameLoaderClient.h:
+        * loader/cache/CachedResourceLoader.cpp:
+        (WebCore::CachedResourceLoader::requestResource):
+        * loader/cache/ResourceLoader.cpp:
+        (WebCore::ResourceLoader::willSendRequestInternal):
+
 2020-02-13  Ryosuke Niwa  <rniwa@webkit.org>
 
         Dark Mode: In Notes, list item becomes invisible in dark mode after outdenting
diff --git a/Source/WebCore/loader/FrameLoaderClient.h b/Source/WebCore/loader/FrameLoaderClient.h
index e9debe1..98d02cf 100644
--- a/Source/WebCore/loader/FrameLoaderClient.h
+++ b/Source/WebCore/loader/FrameLoaderClient.h
@@ -90,6 +90,7 @@
 class Page;
 class PluginViewBase;
 class ProtectionSpace;
+class RegistrableDomain;
 class RTCPeerConnectionHandler;
 class ResourceError;
 class ResourceHandle;
@@ -374,6 +375,7 @@
 
 #if ENABLE(RESOURCE_LOAD_STATISTICS)
     virtual bool hasFrameSpecificStorageAccess() { return false; }
+    virtual void addLoadedRegistrableDomain(RegistrableDomain&&) { }
 #endif
 };
 
diff --git a/Source/WebCore/loader/ResourceLoader.cpp b/Source/WebCore/loader/ResourceLoader.cpp
index a04e3d5..7321700 100644
--- a/Source/WebCore/loader/ResourceLoader.cpp
+++ b/Source/WebCore/loader/ResourceLoader.cpp
@@ -410,9 +410,12 @@
 #endif
 
     bool isRedirect = !redirectResponse.isNull();
-    if (isRedirect)
+    if (isRedirect) {
         platformStrategies()->loaderStrategy()->crossOriginRedirectReceived(this, request.url());
-
+#if ENABLE(RESOURCE_LOAD_STATISTICS)
+        frameLoader()->client().addLoadedRegistrableDomain(RegistrableDomain(request.url()));
+#endif
+    }
     m_request = request;
 
     if (isRedirect) {
diff --git a/Source/WebCore/loader/cache/CachedResourceLoader.cpp b/Source/WebCore/loader/cache/CachedResourceLoader.cpp
index c3d8e35..d42eab4 100644
--- a/Source/WebCore/loader/cache/CachedResourceLoader.cpp
+++ b/Source/WebCore/loader/cache/CachedResourceLoader.cpp
@@ -982,6 +982,9 @@
 
     ASSERT(resource->url() == url.string());
     m_documentResources.set(resource->url(), resource);
+#if ENABLE(RESOURCE_LOAD_STATISTICS)
+    frame.loader().client().addLoadedRegistrableDomain(RegistrableDomain(resource->resourceRequest().url()));
+#endif
     return resource;
 }
 
diff --git a/Source/WebCore/platform/RegistrableDomain.h b/Source/WebCore/platform/RegistrableDomain.h
index 90aec25..565e427 100644
--- a/Source/WebCore/platform/RegistrableDomain.h
+++ b/Source/WebCore/platform/RegistrableDomain.h
@@ -49,6 +49,7 @@
     }
 
     bool isEmpty() const { return m_registrableDomain.isEmpty() || m_registrableDomain == "nullOrigin"_s; }
+    String& string() { return m_registrableDomain; }
     const String& string() const { return m_registrableDomain; }
 
     bool operator!=(const RegistrableDomain& other) const { return m_registrableDomain != other.m_registrableDomain; }
diff --git a/Source/WebKit/ChangeLog b/Source/WebKit/ChangeLog
index 3d0aa94..fb2bade 100644
--- a/Source/WebKit/ChangeLog
+++ b/Source/WebKit/ChangeLog
@@ -1,3 +1,45 @@
+2020-02-13  Kate Cheney  <katherine_cheney@apple.com>
+
+        Expose prevalent domains on a per-page basis
+        https://bugs.webkit.org/show_bug.cgi?id=207523
+        <rdar://problem/59270758>
+
+        Reviewed by Chris Dumez.
+
+        Tests: http/tests/resourceLoadStatistics/prevalent-domains-per-page-database.html
+               http/tests/resourceLoadStatistics/prevalent-domains-per-page.html
+
+        Added an API to report prevalent domains. The Web Process sends all
+        new loads to the Network Process, which determines if the resources are
+        prevalent. When the API is queried, the Web Process reports these
+        domains to the UI Process.
+
+        The WebPage keeps a hash set of previously loaded domains to avoid
+        spamming IPC with repeat domains.
+
+        * NetworkProcess/NetworkConnectionToWebProcess.cpp:
+        (WebKit::NetworkConnectionToWebProcess::requestStorageAccessUnderOpener):
+        (WebKit::NetworkConnectionToWebProcess::reportSubresourceLoadToDomain):
+        * NetworkProcess/NetworkConnectionToWebProcess.h:
+        * NetworkProcess/NetworkConnectionToWebProcess.messages.in:
+        * UIProcess/API/Cocoa/WKWebsiteDataStore.mm:
+        (-[WKWebsiteDataStore _getPrevalentDomainsFor:completionHandler:]):
+        (-[WKWebsiteDataStore _clearPrevalentDomainsFor:]):
+        * UIProcess/API/Cocoa/WKWebsiteDataStorePrivate.h:
+        * UIProcess/WebPageProxy.cpp:
+        (WebKit::WebPageProxy::didCommitLoadForFrame):
+        * UIProcess/WebPageProxy.h:
+        * WebProcess/WebCoreSupport/WebFrameLoaderClient.cpp:
+        (WebKit::WebFrameLoaderClient::addLoadedRegistrableDomain):
+        * WebProcess/WebCoreSupport/WebFrameLoaderClient.h:
+        * WebProcess/WebPage/WebPage.cpp:
+        (WebKit::WebPage::setIsSuspended):
+        (WebKit::WebPage::addLoadedRegistrableDomain):
+        (WebKit::WebPage::getPrevalentDomains):
+        (WebKit::WebPage::clearPrevalentDomains):
+        * WebProcess/WebPage/WebPage.h:
+        * WebProcess/WebPage/WebPage.messages.in:
+
 2020-02-13  Youenn Fablet  <youenn@apple.com>
 
         Protect WebSWServerConnection::scheduleJobInServer from bad scopeURL
diff --git a/Source/WebKit/NetworkProcess/NetworkConnectionToWebProcess.cpp b/Source/WebKit/NetworkProcess/NetworkConnectionToWebProcess.cpp
index e817344..75e0b4a 100644
--- a/Source/WebKit/NetworkProcess/NetworkConnectionToWebProcess.cpp
+++ b/Source/WebKit/NetworkProcess/NetworkConnectionToWebProcess.cpp
@@ -787,6 +787,17 @@
             resourceLoadStatistics->requestStorageAccessUnderOpener(WTFMove(domainInNeedOfStorageAccess), openerPageID, WTFMove(openerDomain));
     }
 }
+
+void NetworkConnectionToWebProcess::isPrevalentSubresourceLoad(RegistrableDomain&& domain, CompletionHandler<void(bool)>&& completionHandler)
+{
+    if (auto* networkSession = this->networkSession()) {
+        if (auto* resourceLoadStatistics = networkSession->resourceLoadStatistics()) {
+            resourceLoadStatistics->isPrevalentResource(domain, WTFMove(completionHandler));
+            return;
+        }
+    }
+    completionHandler(false);
+}
 #endif
 
 void NetworkConnectionToWebProcess::addOriginAccessWhitelistEntry(const String& sourceOrigin, const String& destinationProtocol, const String& destinationHost, bool allowDestinationSubdomains)
diff --git a/Source/WebKit/NetworkProcess/NetworkConnectionToWebProcess.h b/Source/WebKit/NetworkProcess/NetworkConnectionToWebProcess.h
index 86cc60d..eb6f55c6 100644
--- a/Source/WebKit/NetworkProcess/NetworkConnectionToWebProcess.h
+++ b/Source/WebKit/NetworkProcess/NetworkConnectionToWebProcess.h
@@ -258,6 +258,7 @@
     void hasStorageAccess(const RegistrableDomain& subFrameDomain, const RegistrableDomain& topFrameDomain, WebCore::FrameIdentifier, WebCore::PageIdentifier, CompletionHandler<void(bool)>&&);
     void requestStorageAccess(const RegistrableDomain& subFrameDomain, const RegistrableDomain& topFrameDomain, WebCore::FrameIdentifier, WebCore::PageIdentifier, WebPageProxyIdentifier, CompletionHandler<void(WebCore::StorageAccessWasGranted, WebCore::StorageAccessPromptWasShown)>&&);
     void requestStorageAccessUnderOpener(WebCore::RegistrableDomain&& domainInNeedOfStorageAccess, WebCore::PageIdentifier openerPageID, WebCore::RegistrableDomain&& openerDomain);
+    void isPrevalentSubresourceLoad(RegistrableDomain&&, CompletionHandler<void(bool)>&&);
 #endif
 
     void addOriginAccessWhitelistEntry(const String& sourceOrigin, const String& destinationProtocol, const String& destinationHost, bool allowDestinationSubdomains);
diff --git a/Source/WebKit/NetworkProcess/NetworkConnectionToWebProcess.messages.in b/Source/WebKit/NetworkProcess/NetworkConnectionToWebProcess.messages.in
index 43b5f67..ec9b0b8 100644
--- a/Source/WebKit/NetworkProcess/NetworkConnectionToWebProcess.messages.in
+++ b/Source/WebKit/NetworkProcess/NetworkConnectionToWebProcess.messages.in
@@ -62,6 +62,7 @@
     HasStorageAccess(WebCore::RegistrableDomain subFrameDomain, WebCore::RegistrableDomain topFrameDomain, WebCore::FrameIdentifier frameID, WebCore::PageIdentifier pageID) -> (bool hasStorageAccess) Async
     RequestStorageAccess(WebCore::RegistrableDomain subFrameDomain, WebCore::RegistrableDomain topFrameDomain, WebCore::FrameIdentifier frameID, WebCore::PageIdentifier webPageID, WebKit::WebPageProxyIdentifier webPageProxyID) -> (enum:bool WebCore::StorageAccessWasGranted wasGranted, enum:bool WebCore::StorageAccessPromptWasShown promptWasShown) Async
     RequestStorageAccessUnderOpener(WebCore::RegistrableDomain domainInNeedOfStorageAccess, WebCore::PageIdentifier openerPageID, WebCore::RegistrableDomain openerDomain)
+    IsPrevalentSubresourceLoad(WebCore::RegistrableDomain domain) -> (bool isPrevalent) Async
 #endif
 
     AddOriginAccessWhitelistEntry(String sourceOrigin, String destinationProtocol, String destinationHost, bool allowDestinationSubdomains);
diff --git a/Source/WebKit/UIProcess/API/Cocoa/WKWebsiteDataStore.mm b/Source/WebKit/UIProcess/API/Cocoa/WKWebsiteDataStore.mm
index 8667542..432c207c 100644
--- a/Source/WebKit/UIProcess/API/Cocoa/WKWebsiteDataStore.mm
+++ b/Source/WebKit/UIProcess/API/Cocoa/WKWebsiteDataStore.mm
@@ -407,6 +407,46 @@
     WebKit::WebsiteDataStore::allowWebsiteDataRecordsForAllOrigins();
 }
 
+- (void)_getPrevalentDomainsFor:(WKWebView *)webView completionHandler:(void (^)(NSArray<NSString *> *domains))completionHandler
+{
+#if ENABLE(RESOURCE_LOAD_STATISTICS)
+    if (!webView) {
+        completionHandler(nil);
+        return;
+    }
+
+    auto webPageProxy = [webView _page];
+    if (!webPageProxy) {
+        completionHandler(nil);
+        return;
+    }
+    
+    webPageProxy->getPrevalentDomains([completionHandler = makeBlockPtr(completionHandler)] (Vector<WebCore::RegistrableDomain>&& prevalentDomains) {
+        Vector<RefPtr<API::Object>> apiDomains = WTF::map(prevalentDomains, [](auto& domain) {
+            return RefPtr<API::Object>(API::String::create(WTFMove(domain.string())));
+        });
+        completionHandler(wrapper(API::Array::create(WTFMove(apiDomains))));
+    });
+#else
+    completionHandler(nil);
+#endif
+}
+
+- (void)_clearPrevalentDomainsFor:(WKWebView *)webView
+{
+#if ENABLE(RESOURCE_LOAD_STATISTICS)
+    if (!webView)
+        return;
+
+    auto webPageProxy = [webView _page];
+    if (!webPageProxy)
+        return;
+
+    webPageProxy->clearPrevalentDomains();
+#endif
+}
+
+
 - (void)_getAllStorageAccessEntriesFor:(WKWebView *)webView completionHandler:(void (^)(NSArray<NSString *> *domains))completionHandler
 {
     if (!webView) {
diff --git a/Source/WebKit/UIProcess/API/Cocoa/WKWebsiteDataStorePrivate.h b/Source/WebKit/UIProcess/API/Cocoa/WKWebsiteDataStorePrivate.h
index 6e99619..7d3fd5f 100644
--- a/Source/WebKit/UIProcess/API/Cocoa/WKWebsiteDataStorePrivate.h
+++ b/Source/WebKit/UIProcess/API/Cocoa/WKWebsiteDataStorePrivate.h
@@ -62,6 +62,8 @@
 
 - (void)_setResourceLoadStatisticsTestingCallback:(nullable void (^)(WKWebsiteDataStore *, NSString *))callback WK_API_AVAILABLE(macos(10.13), ios(11.0));
 - (void)_getAllStorageAccessEntriesFor:(WKWebView *)webView completionHandler:(void (^)(NSArray<NSString *> *domains))completionHandler WK_API_AVAILABLE(macos(10.14), ios(12.0));
+- (void)_getPrevalentDomainsFor:(WKWebView *)webView completionHandler:(void (^)(NSArray<NSString *> *domains))completionHandler WK_API_AVAILABLE(macos(WK_MAC_TBA), ios(WK_IOS_TBA));
+- (void)_clearPrevalentDomainsFor:(WKWebView *)webView WK_API_AVAILABLE(macos(WK_MAC_TBA), ios(WK_IOS_TBA));
 + (void)_allowWebsiteDataRecordsForAllOrigins WK_API_AVAILABLE(macos(10.13.4), ios(11.3));
 - (bool)_hasRegisteredServiceWorker WK_API_AVAILABLE(macos(10.14), ios(12.0));
 
diff --git a/Source/WebKit/UIProcess/WebPageProxy.cpp b/Source/WebKit/UIProcess/WebPageProxy.cpp
index 7522ef1..163b8f7 100644
--- a/Source/WebKit/UIProcess/WebPageProxy.cpp
+++ b/Source/WebKit/UIProcess/WebPageProxy.cpp
@@ -9856,6 +9856,18 @@
 #endif
 }
 
+#if ENABLE(RESOURCE_LOAD_STATISTICS)
+void WebPageProxy::getPrevalentDomains(CompletionHandler<void(Vector<RegistrableDomain>&&)>&& completionHandler)
+{
+    sendWithAsyncReply(Messages::WebPage::GetPrevalentDomains(), WTFMove(completionHandler));
+}
+
+void WebPageProxy::clearPrevalentDomains()
+{
+    send(Messages::WebPage::ClearPrevalentDomains());
+}
+#endif
+
 } // namespace WebKit
 
 #undef MERGE_WHEEL_EVENTS
diff --git a/Source/WebKit/UIProcess/WebPageProxy.h b/Source/WebKit/UIProcess/WebPageProxy.h
index 7dfcfa3..5ff6792 100644
--- a/Source/WebKit/UIProcess/WebPageProxy.h
+++ b/Source/WebKit/UIProcess/WebPageProxy.h
@@ -1546,6 +1546,8 @@
 #if ENABLE(RESOURCE_LOAD_STATISTICS)
     void requestStorageAccessConfirm(const WebCore::RegistrableDomain& subFrameDomain, const WebCore::RegistrableDomain& topFrameDomain, WebCore::FrameIdentifier, CompletionHandler<void(bool)>&&);
     void didCommitCrossSiteLoadWithDataTransferFromPrevalentResource();
+    void getPrevalentDomains(CompletionHandler<void(Vector<WebCore::RegistrableDomain>&&)>&&);
+    void clearPrevalentDomains();
 #endif
 
 #if ENABLE(DEVICE_ORIENTATION)
diff --git a/Source/WebKit/WebProcess/WebCoreSupport/WebFrameLoaderClient.cpp b/Source/WebKit/WebProcess/WebCoreSupport/WebFrameLoaderClient.cpp
index 121cdca..cfb3d43 100644
--- a/Source/WebKit/WebProcess/WebCoreSupport/WebFrameLoaderClient.cpp
+++ b/Source/WebKit/WebProcess/WebCoreSupport/WebFrameLoaderClient.cpp
@@ -153,6 +153,16 @@
 
     m_frameSpecificStorageAccessIdentifier = WTFMove(frameSpecificStorageAccessIdentifier);
 }
+
+void WebFrameLoaderClient::addLoadedRegistrableDomain(RegistrableDomain&& domain)
+{
+    auto* webPage = m_frame->page();
+    if (!webPage)
+        return;
+    
+    webPage->addLoadedRegistrableDomain(WTFMove(domain));
+}
+
 #endif
 
 void WebFrameLoaderClient::frameLoaderDestroyed()
diff --git a/Source/WebKit/WebProcess/WebCoreSupport/WebFrameLoaderClient.h b/Source/WebKit/WebProcess/WebCoreSupport/WebFrameLoaderClient.h
index 9aa5a05..e377b81 100644
--- a/Source/WebKit/WebProcess/WebCoreSupport/WebFrameLoaderClient.h
+++ b/Source/WebKit/WebProcess/WebCoreSupport/WebFrameLoaderClient.h
@@ -61,6 +61,7 @@
         WebCore::PageIdentifier pageID;
     };
     void setHasFrameSpecificStorageAccess(FrameSpecificStorageAccessIdentifier&&);
+    void addLoadedRegistrableDomain(WebCore::RegistrableDomain&&) final;
 #endif
     
 private:
diff --git a/Source/WebKit/WebProcess/WebPage/WebPage.cpp b/Source/WebKit/WebProcess/WebPage/WebPage.cpp
index e0284a8..aa99e9d 100644
--- a/Source/WebKit/WebProcess/WebPage/WebPage.cpp
+++ b/Source/WebKit/WebProcess/WebPage/WebPage.cpp
@@ -5830,6 +5830,10 @@
     if (!frame->isMainFrame())
         return;
 
+#if ENABLE(RESOURCE_LOAD_STATISTICS)
+    clearPrevalentDomains();
+#endif
+    
     // If previous URL is invalid, then it's not a real page that's being navigated away from.
     // Most likely, this is actually the first load to be committed in this page.
     if (frame->coreFrame()->loader().previousURL().isValid())
@@ -6660,6 +6664,29 @@
 
     frame->document()->wasLoadedWithDataTransferFromPrevalentResource();
 }
+
+void WebPage::addLoadedRegistrableDomain(RegistrableDomain&& domain)
+{
+    auto addResult = m_loadedDomains.add(domain);
+    if (addResult.isNewEntry) {
+        WebProcess::singleton().ensureNetworkProcessConnection().connection().sendWithAsyncReply(Messages::NetworkConnectionToWebProcess::IsPrevalentSubresourceLoad(domain), [this, protectedThis = makeRef(*this), domain] (bool isPrevalent) {
+            if (isPrevalent)
+                m_prevalentDomains.add(domain);
+        });
+    }
+}
+
+void WebPage::getPrevalentDomains(CompletionHandler<void(Vector<RegistrableDomain>)>&& completionHandler)
+{
+    completionHandler(copyToVector(m_prevalentDomains));
+}
+
+void WebPage::clearPrevalentDomains()
+{
+    m_prevalentDomains.clear();
+    m_loadedDomains.clear();
+}
+
 #endif
 
 #if ENABLE(DEVICE_ORIENTATION)
diff --git a/Source/WebKit/WebProcess/WebPage/WebPage.h b/Source/WebKit/WebProcess/WebPage/WebPage.h
index bdab81b2..76d04a0 100644
--- a/Source/WebKit/WebProcess/WebPage/WebPage.h
+++ b/Source/WebKit/WebProcess/WebPage/WebPage.h
@@ -1176,6 +1176,9 @@
     bool hasPageLevelStorageAccess(const WebCore::RegistrableDomain& topLevelDomain, const WebCore::RegistrableDomain& resourceDomain) const;
     void addDomainWithPageLevelStorageAccess(const WebCore::RegistrableDomain& topLevelDomain, const WebCore::RegistrableDomain& resourceDomain);
     void wasLoadedWithDataTransferFromPrevalentResource();
+    void addLoadedRegistrableDomain(WebCore::RegistrableDomain&&);
+    void clearPrevalentDomains();
+    void getPrevalentDomains(CompletionHandler<void(Vector<WebCore::RegistrableDomain>)>&&);
 #endif
 
 #if ENABLE(DEVICE_ORIENTATION)
@@ -2045,6 +2048,8 @@
 
 #if ENABLE(RESOURCE_LOAD_STATISTICS)
     HashMap<WebCore::RegistrableDomain, WebCore::RegistrableDomain> m_domainsWithPageLevelStorageAccess;
+    HashSet<WebCore::RegistrableDomain> m_loadedDomains;
+    HashSet<WebCore::RegistrableDomain> m_prevalentDomains;
 #endif
 
     String m_overriddenMediaType;
diff --git a/Source/WebKit/WebProcess/WebPage/WebPage.messages.in b/Source/WebKit/WebProcess/WebPage/WebPage.messages.in
index 1b4618b..d016e41 100644
--- a/Source/WebKit/WebProcess/WebPage/WebPage.messages.in
+++ b/Source/WebKit/WebProcess/WebPage/WebPage.messages.in
@@ -582,6 +582,8 @@
 
 #if ENABLE(RESOURCE_LOAD_STATISTICS)
     WasLoadedWithDataTransferFromPrevalentResource()
+    ClearPrevalentDomains()
+    GetPrevalentDomains() -> (Vector<WebCore::RegistrableDomain> domains) Async
 #endif
 
 #if USE(SYSTEM_PREVIEW)
diff --git a/Tools/ChangeLog b/Tools/ChangeLog
index 2a00691..f01477b 100644
--- a/Tools/ChangeLog
+++ b/Tools/ChangeLog
@@ -1,3 +1,35 @@
+2020-02-13  Kate Cheney  <katherine_cheney@apple.com>
+
+         Expose prevalent domains on a per-page basis
+         https://bugs.webkit.org/show_bug.cgi?id=207523
+         <rdar://problem/59270758>
+
+        Reviewed by Chris Dumez.
+
+        Added WebKitTestRunner functions so this functionality can be tested.
+        clearPrevalentDomains should be called between tests to reset the
+        state.
+
+        * WebKitTestRunner/InjectedBundle/Bindings/TestRunner.idl:
+        * WebKitTestRunner/InjectedBundle/InjectedBundle.cpp:
+        (WTR::InjectedBundle::didReceiveMessageToPage):
+        * WebKitTestRunner/InjectedBundle/TestRunner.cpp:
+        (WTR::TestRunner::getPrevalentDomains):
+        (WTR::TestRunner::callDidReceivePrevalentDomainsCallback):
+        * WebKitTestRunner/InjectedBundle/TestRunner.h:
+        * WebKitTestRunner/TestController.cpp:
+        (WTR::TestController::resetStateToConsistentValues):
+        (WTR::TestController::getPrevalentDomains):
+        (WTR::TestController::clearPrevalentDomains):
+        * WebKitTestRunner/TestController.h:
+        * WebKitTestRunner/TestInvocation.cpp:
+        (WTR::TestInvocation::didReceiveMessageFromInjectedBundle):
+        (WTR::TestInvocation::didReceivePrevalentDomains):
+        * WebKitTestRunner/TestInvocation.h:
+        * WebKitTestRunner/cocoa/TestControllerCocoa.mm:
+        (WTR::TestController::getPrevalentDomains):
+        (WTR::TestController::clearPrevalentDomains):
+
 2020-02-13  Fujii Hironori  <Hironori.Fujii@sony.com>
 
         [Win][MiniBrowser] Reimplement the toolbar by using toolbar common control
diff --git a/Tools/WebKitTestRunner/InjectedBundle/Bindings/TestRunner.idl b/Tools/WebKitTestRunner/InjectedBundle/Bindings/TestRunner.idl
index 160c2e9..a7fd21e 100644
--- a/Tools/WebKitTestRunner/InjectedBundle/Bindings/TestRunner.idl
+++ b/Tools/WebKitTestRunner/InjectedBundle/Bindings/TestRunner.idl
@@ -344,6 +344,7 @@
     void setStatisticsShouldDowngradeReferrer(boolean value, object callback);
     void setStatisticsShouldBlockThirdPartyCookies(boolean value, object callback, optional boolean onlyOnSitesWithoutUserInteraction);
     void setStatisticsFirstPartyWebsiteDataRemovalMode(boolean value, object callback);
+    void getPrevalentDomains(object callback);
 
     // Injected bundle form client.
     void installTextDidChangeInTextFieldCallback(object callback);
diff --git a/Tools/WebKitTestRunner/InjectedBundle/InjectedBundle.cpp b/Tools/WebKitTestRunner/InjectedBundle/InjectedBundle.cpp
index faedaf5..958a79b 100644
--- a/Tools/WebKitTestRunner/InjectedBundle/InjectedBundle.cpp
+++ b/Tools/WebKitTestRunner/InjectedBundle/InjectedBundle.cpp
@@ -404,6 +404,24 @@
         m_testRunner->callDidReceiveAllStorageAccessEntriesCallback(domains);
         return;
     }
+    
+    if (WKStringIsEqualToUTF8CString(messageName, "CallDidReceivePrevalentDomains")) {
+        ASSERT(messageBody);
+        ASSERT(WKGetTypeID(messageBody) == WKArrayGetTypeID());
+
+        WKArrayRef domainsArray = static_cast<WKArrayRef>(messageBody);
+        auto size = WKArrayGetSize(domainsArray);
+        Vector<String> domains;
+        domains.reserveInitialCapacity(size);
+        for (size_t i = 0; i < size; ++i) {
+            WKTypeRef item = WKArrayGetItemAtIndex(domainsArray, i);
+            if (item && WKGetTypeID(item) == WKStringGetTypeID())
+                domains.uncheckedAppend(toWTFString(static_cast<WKStringRef>(item)));
+        }
+
+        m_testRunner->callDidReceivePrevalentDomainsCallback(WTFMove(domains));
+        return;
+    }
 
     if (WKStringIsEqualToUTF8CString(messageName, "CallDidRemoveAllSessionCredentialsCallback")) {
         m_testRunner->callDidRemoveAllSessionCredentialsCallback();
diff --git a/Tools/WebKitTestRunner/InjectedBundle/TestRunner.cpp b/Tools/WebKitTestRunner/InjectedBundle/TestRunner.cpp
index 24569c9..521f5e5 100644
--- a/Tools/WebKitTestRunner/InjectedBundle/TestRunner.cpp
+++ b/Tools/WebKitTestRunner/InjectedBundle/TestRunner.cpp
@@ -754,6 +754,7 @@
     StatisticsDidSetShouldBlockThirdPartyCookiesCallbackID,
     StatisticsDidSetFirstPartyWebsiteDataRemovalModeCallbackID,
     AllStorageAccessEntriesCallbackID,
+    GetPrevalentDomainsCallbackID,
     DidRemoveAllSessionCredentialsCallbackID,
     GetApplicationManifestCallbackID,
     TextDidChangeInTextFieldCallbackID,
@@ -2352,6 +2353,38 @@
     callTestRunnerCallback(AllStorageAccessEntriesCallbackID, 1, &result);
 }
 
+void TestRunner::getPrevalentDomains(JSValueRef callback)
+{
+    cacheTestRunnerCallback(GetPrevalentDomainsCallbackID, callback);
+    
+    WKRetainPtr<WKStringRef> messageName = adoptWK(WKStringCreateWithUTF8CString("GetPrevalentDomains"));
+    WKBundlePostMessage(InjectedBundle::singleton().bundle(), messageName.get(), nullptr);
+}
+
+void TestRunner::callDidReceivePrevalentDomainsCallback(Vector<String>&& domains)
+{
+    WKBundleFrameRef mainFrame = WKBundlePageGetMainFrame(InjectedBundle::singleton().page()->page());
+    JSContextRef context = WKBundleFrameGetJavaScriptContext(mainFrame);
+    
+    StringBuilder stringBuilder;
+    stringBuilder.append('[');
+    bool isFirstDomain = true;
+    for (auto& domain : domains) {
+        if (isFirstDomain)
+            isFirstDomain = false;
+        else
+            stringBuilder.appendLiteral(", ");
+        stringBuilder.appendLiteral("\"");
+        stringBuilder.append(domain);
+        stringBuilder.appendLiteral("\"");
+    }
+    stringBuilder.append(']');
+    
+    JSValueRef result = JSValueMakeFromJSONString(context, adopt(JSStringCreateWithUTF8CString(stringBuilder.toString().utf8().data())).get());
+
+    callTestRunnerCallback(GetPrevalentDomainsCallbackID, 1, &result);
+}
+
 void TestRunner::addMockMediaDevice(JSStringRef persistentId, JSStringRef label, const char* type)
 {
     Vector<WKRetainPtr<WKStringRef>> keys;
diff --git a/Tools/WebKitTestRunner/InjectedBundle/TestRunner.h b/Tools/WebKitTestRunner/InjectedBundle/TestRunner.h
index c3c1045..7228b5b 100644
--- a/Tools/WebKitTestRunner/InjectedBundle/TestRunner.h
+++ b/Tools/WebKitTestRunner/InjectedBundle/TestRunner.h
@@ -448,6 +448,8 @@
     void statisticsCallDidSetFirstPartyWebsiteDataRemovalModeCallback();
     void statisticsResetToConsistentState(JSValueRef completionHandler);
     void statisticsCallDidResetToConsistentStateCallback();
+    void getPrevalentDomains(JSValueRef callback);
+    void callDidReceivePrevalentDomainsCallback(Vector<String>&& domains);
 
     // Injected bundle form client.
     void installTextDidChangeInTextFieldCallback(JSValueRef callback);
diff --git a/Tools/WebKitTestRunner/TestController.cpp b/Tools/WebKitTestRunner/TestController.cpp
index bf14227..5cd9237 100644
--- a/Tools/WebKitTestRunner/TestController.cpp
+++ b/Tools/WebKitTestRunner/TestController.cpp
@@ -1109,7 +1109,8 @@
     setAllowsAnySSLCertificate(true);
 
     statisticsResetToConsistentState();
-    
+    clearPrevalentDomains();
+
     clearAdClickAttribution();
 
     m_didReceiveServerRedirectForProvisionalNavigation = false;
@@ -3113,6 +3114,14 @@
 {
 }
 
+void TestController::getPrevalentDomains()
+{
+}
+
+void TestController::clearPrevalentDomains()
+{
+}
+
 #endif
 
 struct ClearServiceWorkerRegistrationsCallbackContext {
diff --git a/Tools/WebKitTestRunner/TestController.h b/Tools/WebKitTestRunner/TestController.h
index fa60882..4e65112 100644
--- a/Tools/WebKitTestRunner/TestController.h
+++ b/Tools/WebKitTestRunner/TestController.h
@@ -258,6 +258,8 @@
     void statisticsResetToConsistentState();
 
     void getAllStorageAccessEntries();
+    void getPrevalentDomains();
+    void clearPrevalentDomains();
 
     WKArrayRef openPanelFileURLs() const { return m_openPanelFileURLs.get(); }
     void setOpenPanelFileURLs(WKArrayRef fileURLs) { m_openPanelFileURLs = fileURLs; }
diff --git a/Tools/WebKitTestRunner/TestInvocation.cpp b/Tools/WebKitTestRunner/TestInvocation.cpp
index 29b99f8..81afb52 100644
--- a/Tools/WebKitTestRunner/TestInvocation.cpp
+++ b/Tools/WebKitTestRunner/TestInvocation.cpp
@@ -807,6 +807,11 @@
         return;
     }
 #endif
+    
+    if (WKStringIsEqualToUTF8CString(messageName, "GetPrevalentDomains")) {
+        TestController::singleton().getPrevalentDomains();
+        return;
+    }
 
     ASSERT_NOT_REACHED();
 }
@@ -1942,6 +1947,17 @@
     WKPagePostMessageToInjectedBundle(TestController::singleton().mainWebView()->page(), messageName.get(), messageBody.get());
 }
 
+void TestInvocation::didReceivePrevalentDomains(Vector<String>&& domains)
+{
+    WKRetainPtr<WKStringRef> messageName = adoptWK(WKStringCreateWithUTF8CString("CallDidReceivePrevalentDomains"));
+    
+    WKRetainPtr<WKMutableArrayRef> messageBody = adoptWK(WKMutableArrayCreate());
+    for (auto& domain : domains)
+        WKArrayAppendItem(messageBody.get(), adoptWK(WKStringCreateWithUTF8CString(domain.utf8().data())).get());
+    
+    WKPagePostMessageToInjectedBundle(TestController::singleton().mainWebView()->page(), messageName.get(), messageBody.get());
+}
+
 void TestInvocation::didRemoveAllSessionCredentials()
 {
     WKRetainPtr<WKStringRef> messageName = adoptWK(WKStringCreateWithUTF8CString("CallDidRemoveAllSessionCredentialsCallback"));
diff --git a/Tools/WebKitTestRunner/TestInvocation.h b/Tools/WebKitTestRunner/TestInvocation.h
index e1c2c5d..50337d6 100644
--- a/Tools/WebKitTestRunner/TestInvocation.h
+++ b/Tools/WebKitTestRunner/TestInvocation.h
@@ -86,6 +86,7 @@
     void didSetVeryPrevalentResource();
     void didSetHasHadUserInteraction();
     void didReceiveAllStorageAccessEntries(Vector<String>& domains);
+    void didReceivePrevalentDomains(Vector<String>&& domains);
 
     void didRemoveAllSessionCredentials();
     
diff --git a/Tools/WebKitTestRunner/cocoa/TestControllerCocoa.mm b/Tools/WebKitTestRunner/cocoa/TestControllerCocoa.mm
index 3f9304e..81c0a06 100644
--- a/Tools/WebKitTestRunner/cocoa/TestControllerCocoa.mm
+++ b/Tools/WebKitTestRunner/cocoa/TestControllerCocoa.mm
@@ -332,6 +332,30 @@
     }];
 }
 
+void TestController::getPrevalentDomains()
+{
+    auto* parentView = mainWebView();
+    if (!parentView)
+        return;
+    
+    [globalWebViewConfiguration.websiteDataStore _getPrevalentDomainsFor:parentView->platformView() completionHandler:^(NSArray<NSString *> *nsDomains) {
+        Vector<String> domains;
+        domains.reserveInitialCapacity(nsDomains.count);
+        for (NSString *domain : nsDomains)
+            domains.uncheckedAppend(domain);
+        m_currentInvocation->didReceivePrevalentDomains(WTFMove(domains));
+    }];
+}
+
+void TestController::clearPrevalentDomains()
+{
+    auto* parentView = mainWebView();
+    if (!parentView)
+        return;
+
+    [globalWebViewConfiguration.websiteDataStore _clearPrevalentDomainsFor:parentView->platformView()];
+}
+
 void TestController::injectUserScript(WKStringRef script)
 {
     auto userScript = adoptNS([[WKUserScript alloc] initWithSource: toWTFString(script) injectionTime:WKUserScriptInjectionTimeAtDocumentStart forMainFrameOnly:NO]);