Refactor backdrop that is currently used in the maximized mode.

ash/test/workspace_controller_test_api.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/workspace_controller_test_api.h"
 
 #include "ash/test/workspace_event_handler_test_helper.h"
+#include "ash/wm/workspace/backdrop_controller.h"
+#include "ash/wm/workspace/workspace_layout_manager.h"
 #include "ash/wm/workspace_controller.h"
 #include "ui/aura/window.h"
 
 namespace ash {
 namespace test {
 
 WorkspaceControllerTestApi::WorkspaceControllerTestApi(
     WorkspaceController* controller)
     : controller_(controller) {}
 
 WorkspaceControllerTestApi::~WorkspaceControllerTestApi() {}
 
 WorkspaceEventHandler* WorkspaceControllerTestApi::GetEventHandler() {
   return controller_->event_handler_.get();
 }
 
 MultiWindowResizeController*
 WorkspaceControllerTestApi::GetMultiWindowResizeController() {
   return WorkspaceEventHandlerTestHelper(GetEventHandler()).resize_controller();
 }
 
+aura::Window* WorkspaceControllerTestApi::GetBackdropWindow() {
+  return controller_->layout_manager_->backdrop_controller_->backdrop_window_;
+}
+
 }  // namespace test
 }  // namespace ash