third_party/WebKit/LayoutTests/fast/loader/unload-form-post-about-blank.html

<script>
addEventListener("unload", function() {
  document.getElementById("fm").submit();
}, false);
</script>
<form id="fm" action="about:blank" method="POST"></form>
<script>
if (window.testRunner) {
  testRunner.dumpAsText();
  testRunner.waitUntilDone();
}
window.location = "resources/pass-and-notify-done.html";
</script>