mash: Remove WmRootWindowControllerObserver.

ash/mus/bridge/wm_root_window_controller_mus.h

 // Copyright 2016 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_MUS_BRIDGE_WM_ROOT_WINDOW_CONTROLLER_MUS_H_
 #define ASH_MUS_BRIDGE_WM_ROOT_WINDOW_CONTROLLER_MUS_H_
 
 #include "ash/common/wm_root_window_controller.h"
 #include "base/macros.h"
 
@@ skipping 48 matching lines @@
       int shell_container_id,
       views::Widget::InitParams* init_params) override;
   WmWindow* FindEventTarget(const gfx::Point& location_in_screen) override;
   gfx::Point GetLastMouseLocationInRoot() override;
 
  private:
   friend class RootWindowController;
 
   WmShellMus* shell_;
   RootWindowController* root_window_controller_;
-  base::ObserverList<WmRootWindowControllerObserver> observers_;
 
   DISALLOW_COPY_AND_ASSIGN(WmRootWindowControllerMus);
 };
 
 }  // namespace mus
 }  // namespace ash
 
 #endif  // ASH_MUS_BRIDGE_WM_ROOT_WINDOW_CONTROLLER_MUS_H_