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chromium
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chromium/third_party/blink/web_tests/http/tests/security/resources/agent-equality-report-to-parent.html 

<!DOCTYPE html>
<html>
<head>
<title>Agent ID reporter frame</title>
</head>
<body>
<script>
window.parent.postMessage(internals.getAgentId(window), '*');
</script>
</body>
</html>