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

ash/drag_drop/drag_drop_controller.h

 // 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.
 
 #ifndef ASH_DRAG_DROP_DRAG_DROP_CONTROLLER_H_
 #define ASH_DRAG_DROP_DRAG_DROP_CONTROLLER_H_
 #pragma once
 
 #include "ash/ash_export.h"
+#include "base/callback.h"
 #include "ui/aura/client/drag_drop_client.h"
 #include "ui/aura/event.h"
 #include "ui/aura/event_filter.h"
 #include "ui/aura/window_observer.h"
 #include "ui/base/dragdrop/os_exchange_data.h"
 #include "ui/base/events.h"
 #include "ui/compositor/layer_animation_observer.h"
 #include "ui/gfx/point.h"
 
 namespace aura {
@@ skipping 72 matching lines @@
   // Window that is currently under the drag cursor.
   aura::Window* drag_window_;
   gfx::Point drag_start_location_;
 
   bool drag_drop_in_progress_;
 
   // Indicates whether the caller should be blocked on a drag/drop session.
   // Only be used for tests.
   bool should_block_during_drag_drop_;
 
+  // Closure for quitting nested message loop.
+  base::Closure quit_closure_;
+
   DISALLOW_COPY_AND_ASSIGN(DragDropController);
 };
 
 }  // namespace internal
 }  // namespace ash
 
 #endif  // ASH_DRAG_DROP_DRAG_DROP_CONTROLLER_H_