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chromium/third_party/blink/web_tests/http/tests/websocket/resources/iframe-sandbox-iframe.html 

<!DOCTYPE html>
<script src="get-request-header.js"></script>
<script>
connectAndGetRequestHeader('origin').then(function(origin)
{
    window.parent.postMessage(origin, "*");
}, function() { window.parent.postMessage("FAIL", "*"); });
</script>