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chromium/third_party/blink/web_tests/http/tests/misc/reentrant-beforeunload.html 

<html>
<body>
<script>
if (window.testRunner) {
    testRunner.dumpAsText();
    testRunner.waitUntilDone();
    testRunner.setPopupBlockingEnabled(false);
}

var w = window.open("resources/reentrant-beforeunload-helper.html");
w.onload = function() {
    setTimeout(function() {
        w.close();
        if (window.testRunner)
            testRunner.notifyDone();
    }, 0);
}; 
</script>
</body>
</html>