mash: Relay some Chrome aura window properties to Ash.

ui/views/mus/mus_client_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 UI_VIEWS_MUS_MUS_CLIENT_OBSERVER_H_
 #define UI_VIEWS_MUS_MUS_CLIENT_OBSERVER_H_
 
 #include "ui/views/mus/mus_export.h"
 
 namespace views {
 
 // Allows individual DesktopWindowTreeHostsMus to observe MusClient events.
 class VIEWS_MUS_EXPORT MusClientObserver {
  public:
   // Notification that windows should update per window values from the
   // WindowManagerFrameValues.
   virtual void OnWindowManagerFrameValuesChanged() = 0;
 
+  // Notification that a window property value has changed.
+  virtual void OnWindowPropertyChanged(aura::Window* window,
+                                       const void* key) = 0;
+
  protected:
   virtual ~MusClientObserver() {}
 };
 
 }  // namespace views
 
 #endif  // UI_VIEWS_MUS_MUS_CLIENT_OBSERVER_H_