Add base::RunLoop and update ui_test_utils to use it to reduce flakiness

chrome/test/base/test_web_dialog_observer.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/test/base/test_web_dialog_observer.h"
 
 #include "chrome/test/base/ui_test_utils.h"
 #include "content/public/browser/notification_service.h"
 #include "content/public/browser/navigation_controller.h"
 #include "content/public/browser/notification_details.h"
@@ skipping 24 matching lines @@
       break;
     case content::NOTIFICATION_LOAD_STOP:
       DCHECK(web_ui_);
       registrar_.Remove(this, content::NOTIFICATION_LOAD_STOP,
                         content::Source<NavigationController>(
                             &web_ui_->GetWebContents()->GetController()));
       done_ = true;
       // If the message loop is running stop it.
       if (running_) {
         running_ = false;
-        MessageLoopForUI::current()->Quit();
+        message_loop_runner_->Quit();
       }
       break;
     default:
       NOTREACHED();
   };
 }
 
 void TestWebDialogObserver::OnDialogShown(
       content::WebUI* webui,
       content::RenderViewHost* render_view_host) {
@@ skipping 10 matching lines @@
   // navigate in this method, ensuring that this is not a race condition.
   registrar_.Add(this, content::NOTIFICATION_LOAD_STOP,
                  content::Source<NavigationController>(
                      &web_ui_->GetWebContents()->GetController()));
 }
 
 content::WebUI* TestWebDialogObserver::GetWebUI() {
   if (!done_) {
     EXPECT_FALSE(running_);
     running_ = true;
-    ui_test_utils::RunMessageLoop();
+    message_loop_runner_ = new ui_test_utils::MessageLoopRunner;
+    message_loop_runner_->Run();
   }
   return web_ui_;
 }