LayoutTests/http/tests/site-isolation/load-event.html

<!-- webkit-test-runner [ SiteIsolationEnabled=true ] -->
<script src="/js-test-resources/js-test.js"></script>
<script>
if (window.testRunner) {
    testRunner.dumpAsText();
    testRunner.waitUntilDone();
}

function test() {
    testPassed("Load event fired on frame's owner element");
    testRunner.notifyDone();
}
</script>
<iframe onload="test()" src="http://localhost:8000/site-isolation/resources/green-background.html"></iframe>