Changes how window manager obtains WindowTree from mus

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) = 0;
+  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_