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chromium
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chromium/third_party/blink/web_tests/http/tests/security/resources/cross-frame-iframe.html 

<html>
<head>
    <script>
        function fireSentinel()
        {
            parent.postMessage("LOADED", "*");
        }
    </script>
</head>
<body onload="fireSentinel();">
    <p id='accessMe'></p>
    <p>Inner iframe.</p>
</body>
</html>