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chromium/third_party/blink/web_tests/external/wpt/html/browsers/origin/origin-keyed-agent-clusters/resources/frame.html 

<!DOCTYPE html>
<meta charset="utf-8">
<title>A frame included by a test page</title>

<script>
window.onmessage = e => {
  if (e.data.type === "set document.domain") {
    document.domain = e.data.newValue;
    e.source.postMessage({ type: "new document.domain", result: document.domain }, "*");
  }
};
</script>