Refactor backdrop that is currently used in the maximized mode.

ash/test/workspace_controller_test_api.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_TEST_WORKSPACE_CONTROLLER_TEST_API_H_
 #define ASH_TEST_WORKSPACE_CONTROLLER_TEST_API_H_
 
 #include "ash/ash_export.h"
 #include "ash/wm/workspace_controller.h"
 #include "base/macros.h"
 
 namespace ash {
 class MultiWindowResizeController;
 class WorkspaceEventHandler;
 
 namespace test {
 
 class ASH_EXPORT WorkspaceControllerTestApi {
  public:
   explicit WorkspaceControllerTestApi(WorkspaceController* controller);
   ~WorkspaceControllerTestApi();
 
   WorkspaceEventHandler* GetEventHandler();
   MultiWindowResizeController* GetMultiWindowResizeController();
+  aura::Window* GetBackdropWindow();
 
  private:
   WorkspaceController* controller_;
 
   DISALLOW_COPY_AND_ASSIGN(WorkspaceControllerTestApi);
 };
 
 }  // namespace test
 }  // namespace ash
 
 #endif  // ASH_TEST_WORKSPACE_CONTROLLER_TEST_API_H_