Limit beforeunload returnValue propagation to event handlers.

LayoutTests/fast/loader/form-submission-after-beforeunload-cancel.html

 <script>
 if (window.testRunner) {
     testRunner.waitUntilDone();
     testRunner.dumpAsText();
     testRunner.setShouldStayOnPageAfterHandlingBeforeUnload(true);
 }
 
 _confirmationDialogDisplayedOnce = false;
 
-window.addEventListener("beforeunload", function() {
+window.onbeforeunload = function() {
 
     if (window._confirmationDialogDisplayedOnce)
         return "Click 'Leave Page'";
 
     window.setTimeout(function() {
         if (window.testRunner)
             testRunner.setShouldStayOnPageAfterHandlingBeforeUnload(false);
 
         window.setTimeout(function() {
             document.getElementById("console").innerHTML = "FAIL";
             if (window.testRunner)
                 testRunner.notifyDone();
         }, 1000);
 
         document.forms[0].submit();
     }, 0);
-    
+
     window._confirmationDialogDisplayedOnce = true;
-    
+
     return "Click 'Stay on Page'";
-});
+};
 </script>
 
 <p>This tests that submitting a form a second time after canceling the first submission in a onbeforeunload handler is allowed. To test manually, follow the instructions in the JavaScript confirmation dialogs.</p>
 
 <div id="console"></div>
 
 <form action="resources/pass-and-notify-done.html" method="POST">
 </form>
 
 <script>
 document.forms[0].submit();
-</script>
+</script>