Converts mash to use Shell

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) {}
-
- protected:
-  virtual ~WindowManagerObserver() {}
-};
-
-}  // namespace mus
-}  // namespace ash
-
-#endif  // ASH_MUS_WINDOW_OBSERVER_H_