Refactor TimeBase to post tasks. Workers to use real Idle tasks.

LayoutTests/fast/events/resources/subframe-stop-load-in-unload-handler-using-document-write.html

 <script>
-    window.location = "pass.html";
+    // We need to wait until the iframe has finished loading before navigating or
+    // we risk getting duplicate prints from WebTestProxyBase::DidFinishDocumentLoad
+    // due to a race between the FrameLoader::startLoad due to setting window.location
+    // and Document::finishedParsing both of which call FrameLoader::finishedParsing
+    // which in turn calls. WebTestProxyBase::DidFinishDocumentLoad.
+    window.onload = function() {
+        window.location = "pass.html";
+    }
 
     window.onunload = function()
     {
         document.write("document.write");
         setTimeout("parent.log('You wanted to go to: '+ location.href); parent.fail()", 500);
     }
 </script>