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

ash/test/ash_test_base.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 "ash/test/ash_test_base.h"
 
 #include <string>
 #include <vector>
 
 #include "ash/shell.h"
 #include "ash/test/test_shell_delegate.h"
+#include "base/run_loop.h"
 #include "base/string_split.h"
 #include "content/public/test/web_contents_tester.h"
 #include "ui/aura/env.h"
 #include "ui/aura/monitor_manager.h"
 #include "ui/aura/root_window.h"
 #include "ui/base/ime/text_input_test_support.h"
 #include "ui/compositor/layer_animator.h"
 #include "ui/gfx/display.h"
 #include "ui/gfx/screen.h"
 
@@ skipping 61 matching lines @@
 }
 
 void AshTestBase::UpdateMonitor(const std::string& display_specs) {
   std::vector<gfx::Display> displays = CreateDisplaysFromString(display_specs);
   aura::Env::GetInstance()->monitor_manager()->
       OnNativeMonitorsChanged(displays);
 }
 
 void AshTestBase::RunAllPendingInMessageLoop() {
 #if !defined(OS_MACOSX)
-  message_loop_.RunAllPendingWithDispatcher(
-      aura::Env::GetInstance()->GetDispatcher());
+  DCHECK(MessageLoopForUI::current() == &message_loop_);
+  base::RunLoop run_loop(aura::Env::GetInstance()->GetDispatcher());
+  run_loop.RunUntilIdle();
 #endif
 }
 
 }  // namespace test
 }  // namespace ash