mash: Relay some Chrome aura window properties to Ash.

ui/views/mus/mus_client_observer.h

Index: ui/views/mus/mus_client_observer.h
diff --git a/ui/views/mus/mus_client_observer.h b/ui/views/mus/mus_client_observer.h
index 8d633a5b34708554f87f5c74223efe2e8d8e0d59..c619325e1f1787a29f252a51d1c52831c4a39887 100644
--- a/ui/views/mus/mus_client_observer.h
+++ b/ui/views/mus/mus_client_observer.h
@@ -16,6 +16,10 @@ class VIEWS_MUS_EXPORT MusClientObserver {
   // 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() {}
 };