Make aura work with mus

ui/aura/window_observer.h

Index: ui/aura/window_observer.h
diff --git a/ui/aura/window_observer.h b/ui/aura/window_observer.h
index 9a271cf38630acbc0350d82d8195a4bd1c6a25fc..15292fc513a80f3adcbab1b6572895fd618805c3 100644
--- a/ui/aura/window_observer.h
+++ b/ui/aura/window_observer.h
@@ -126,6 +126,10 @@ class AURA_EXPORT WindowObserver {
   // Called when the window title has changed.
   virtual void OnWindowTitleChanged(Window* window) {}
 
+  // Called when the app embedded in |window| disconnects (is no longer
+  // embedded).
+  virtual void OnEmbeddedAppDisconnected(Window* window);
+
  protected:
   virtual ~WindowObserver();