aura-mus: Implement Deactivate().

ui/aura/mus/focus_synchronizer.h

Index: ui/aura/mus/focus_synchronizer.h
diff --git a/ui/aura/mus/focus_synchronizer.h b/ui/aura/mus/focus_synchronizer.h
index db2556d76155f0e1cdb8000f5a0ab6af217f901a..d1483c14e8384616485011d064e2a5be32e0c370 100644
--- a/ui/aura/mus/focus_synchronizer.h
+++ b/ui/aura/mus/focus_synchronizer.h
@@ -36,6 +36,9 @@ class FocusSynchronizer : public client::FocusChangeObserver,
   // Called when the server side wants to change focus to |window|.
   void SetFocusFromServer(WindowMus* window);
 
+  // Called when the client wishes to deactivate itself.
+  void ClearFocus();
+
   // Called when the focused window is destroyed.
   void OnFocusedWindowDestroyed();