Refactor backdrop

ash/wm/maximize_mode/maximize_mode_window_manager.h

 // Copyright 2014 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_WM_MAXIMIZE_MODE_MAXIMIZE_MODE_WINDOW_MANAGER_H_
 #define ASH_WM_MAXIMIZE_MODE_MAXIMIZE_MODE_WINDOW_MANAGER_H_
 
 #include <stdint.h>
 
 #include <map>
@@ skipping 113 matching lines @@
 
   // Every window which got touched by our window manager gets added here.
   WindowToState window_state_map_;
 
   // All container windows which have to be tracked.
   std::unordered_set<aura::Window*> observed_container_windows_;
 
   // Windows added to the container, but not yet shown.
   std::unordered_set<aura::Window*> added_windows_;
 
-  // True if all backdrops are hidden.
-  bool backdrops_hidden_;
-
   std::unique_ptr<wm::MaximizeModeEventHandler> event_handler_;
 
   DISALLOW_COPY_AND_ASSIGN(MaximizeModeWindowManager);
 };
 
 }  // namespace ash
 
 #endif  // ASH_WM_MAXIMIZE_MODE_MAXIMIZE_MODE_WINDOW_MANAGER_H_