Only show a beforeunload dialog if a frame has had a user gesture since its load.

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

 <script>
 if (window.testRunner) {
     testRunner.waitUntilDone();
     testRunner.dumpAsText();
     testRunner.setShouldStayOnPageAfterHandlingBeforeUnload(true);
 }
 
+// JavaScript onbeforeunload dialogs require a user gesture.
+if (window.eventSender) {
+    eventSender.mouseMoveTo(5, 5);
+    eventSender.mouseDown();
+    eventSender.mouseUp();
+}
+
 _confirmationDialogDisplayedOnce = false;
 
 window.onbeforeunload = function() {
 
     if (window._confirmationDialogDisplayedOnce)
         return "Click 'Leave Page'";
 
     window.setTimeout(function() {
         if (window.testRunner)
             testRunner.setShouldStayOnPageAfterHandlingBeforeUnload(false);
@@ skipping 16 matching lines @@
 <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>