LayoutTests/http/tests/security/beforeload-iframe-client-redirect.html

<script>
if (window.testRunner) {
    testRunner.dumpAsText();
    testRunner.waitUntilDone();
}

window.addEventListener('message', function(evt) {
    if (window.testRunner)
        testRunner.notifyDone();
}, false);
</script>
This test produces output in the console.  Only the initial URL should be logged.
<iframe
  src="http://127.0.0.1:8000/security/resources/client-redir.html?url=http://localhost:8000/security/resources/post-done.html"
  onbeforeload="console.log(event.url)">
</iframe>