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chromium/third_party/blink/web_tests/fast/dom/content-document-prototype.html 

<body>
<script>
if (window.testRunner)
    testRunner.dumpAsText();

var ifm = document.createElement('iframe');
document.body.appendChild(ifm);

var parentDocumentPrototype = document.__proto__;
var childDocumentPrototype = ifm.contentDocument.__proto__;

if (parentDocumentPrototype === childDocumentPrototype)
    document.write("FAIL! The parent and child documents have the same prototype.");
else
    document.write("PASS");
</script>