Fixes shutdown race in mash

ash/mus/root_windows_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_ROOT_WINDOW_OBSERVERS_H_
-#define ASH_MUS_ROOT_WINDOW_OBSERVERS_H_
-
-namespace ash {
-namespace mus {
-
-class RootWindowController;
-
-class RootWindowsObserver {
- public:
-  // Called once a new display has been created and the root Window obtained.
-  virtual void OnRootWindowControllerAdded(RootWindowController* controller) {}
-
-  // Called before a RootWindowController is destroyed.
-  virtual void OnWillDestroyRootWindowController(
-      RootWindowController* controller) {}
-
- protected:
-  ~RootWindowsObserver() {}
-};
-
-}  // namespace mus
-}  // namespace ash
-
-#endif  // ASH_MUS_ROOT_WINDOW_OBSERVERS_H_