<!DOCTYPE html>
<script>
  // When told, register an unload handler that will trigger a beacon to the
  // URL given by the sender of the message.
  window.addEventListener('message', e => {
      var url = e.data;
      window.addEventListener('unload', () => {
          navigator.sendBeacon(url, 'blah');
      });
      window.parent.postMessage('navigate-away', '*');
  });
</script>