chromium/third_party/blink/web_tests/fast/frames/grandchild-navigates-child-in-beforeunload.html

<iframe src="resources/grandchild-navigates-child-in-beforeunload-child.html"></iframe>
<script>
if (window.testRunner) {
  testRunner.dumpAsText();
  testRunner.dumpChildFrames();
}

window.onload = () => {
  frames[0][0].location = "resources/pass-and-notify-done.html";
}
</script>