<iframe src="http://does.not.exist/" id="target"></iframe>
<script>
if (window.testRunner) {
    testRunner.dumpAsText();
    testRunner.waitUntilDone();
}

let frame = document.getElementById("target");
frame.contentWindow.onbeforeunload = function() {
    setTimeout(function() {
        frame.src = "javascript:alert('FAIL')";
    }, 0);
};

window.addEventListener("load", function() {
    document.write("PASS - Javascript URL blocked without crashing.");
    if (window.testRunner)
        testRunner.notifyDone();
}, false);
</script>