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chromium/third_party/blink/web_tests/http/tests/security/resources/sandboxed-iframe-form-top.html 

<html>
<body>
<form id="theForm" action="fail.html" method="GET" target="_top">
<input type="submit" value="Submit"/>
</form>
<script>
var f = document.getElementById('theForm');
f.submit();
setTimeout(function() {
    if (window.testRunner)
        testRunner.notifyDone();
}, 0);
</script>
</body>
</html>