messageport-postmessage-iframe.html

<!doctype html>
<html>
  <script src="task-ids.js"></script>
  <script>
    window.addEventListener('message', e => {
      const port = e.ports?.[0];
      initializeTaskId();
      port.postMessage(scheduler.taskId);
    });
  </script>
</html>

chromium/third_party/blink/web_tests/wpt_internal/task-tracking/resources/messageport-postmessage-iframe.html