use-counter-third-party-parent.html

<!DOCTYPE html>
<meta charset="utf-8">
<script src="/resources/get-host-info.js"></script>
<title>Parent</title>
<body></body>
<script>
window.onload = function() {
  window.opener.postMessage('popup_page_loaded', '*');
};

window.onmessage = (msg) => {
  if (msg.data == 'from_opener_assert_has_use_counter') {
    const iframe = document.createElement('iframe');
    iframe.src = get_host_info().HTTP_ORIGIN +
        '/serviceworker/resources/use-counter-third-party-controlled-iframe.html';
    document.body.appendChild(iframe);
    iframe.onload = function() {
      iframe.contentWindow.postMessage(msg.data, '*');
    };
  } else {
    window.opener.postMessage(msg.data, '*');
  }
};
</script>

chromium/third_party/blink/web_tests/http/tests/serviceworker/resources/use-counter-third-party-parent.html