Moves mash/wm into ash/mus

mash/wm/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 MASH_WM_ROOT_WINDOW_OBSERVERS_H_
-#define MASH_WM_ROOT_WINDOW_OBSERVERS_H_
-
-namespace mash {
-namespace wm {
-
-class RootWindowController;
-
-class RootWindowsObserver {
- public:
-  // Called once a new display has been created and the root Window obtained.
-  virtual void OnRootWindowControllerAdded(
-      RootWindowController* controller) = 0;
-
- protected:
-  ~RootWindowsObserver() {}
-};
-
-}  // namespace wm
-}  // namespace mash
-
-#endif  // MASH_WM_ROOT_WINDOW_OBSERVERS_H_