[Async][WIP] Call FrameLoader::checkCompleted() asynchronously to avoid sync body.onload()

third_party/WebKit/LayoutTests/fullscreen/non-ancestor-iframe.html

 <script src="full-screen-test.js"></script>
 <div>This tests that full screen mode will not exit when a non-ancestor iframe is loaded in the page. Click <button onclick="startTest();">enter full screen</button> to start test.</div>
 <div id="target">
     <iframe id="iframe"></iframe>
 </div>
 <script>
     var target = document.getElementById('target');
     var iframe = document.getElementById('iframe');
 
     waitForEventOnce(document, 'webkitfullscreenchange', function() {
         waitForEvent(document, 'webkitfullscreenchange', function() {
-           logResult(false, 'Exited full screen'); 
+            logResult(false, 'Exited full screen'); 
+        });
+        waitForEvent(iframe, 'load', function() {
+            testExpected("document.webkitIsFullScreen", true);
+            endTest();
         });
         iframe.src = 'resources/empty.html';
     });
 
-    waitForEvent(iframe, 'load', function() {
-        testExpected("document.webkitIsFullScreen", true);
-        endTest();
-    });
-
     var startTest = function() {
         target.webkitRequestFullscreen();
     };
 
     if (window.testRunner)
         runWithKeyDown(startTest);
 </script>