Centralizes more shared code between ash and mash

ash/mus/window_manager_observer.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_WINDOW_OBSERVER_H_
 #define ASH_MUS_WINDOW_OBSERVER_H_
 
 namespace ash {
 namespace mus {
 
 class RootWindowController;
 
 class WindowManagerObserver {
  public:
   // Called when the WindowTreeClient associated with the WindowManager is
   // about to be destroyed.
   virtual void OnWindowTreeClientDestroyed() {}
 
   virtual void OnRootWindowControllerAdded(RootWindowController* controller) {}
-  virtual void OnWillDestroyRootWindowController(
-      RootWindowController* controller) {}
 
  protected:
   virtual ~WindowManagerObserver() {}
 };
 
 }  // namespace mus
 }  // namespace ash
 
 #endif  // ASH_MUS_WINDOW_OBSERVER_H_