aura-mus: Implement Deactivate().

ui/aura/mus/window_manager_delegate.h

Index: ui/aura/mus/window_manager_delegate.h
diff --git a/ui/aura/mus/window_manager_delegate.h b/ui/aura/mus/window_manager_delegate.h
index 0dc8d6186712b1d15616733c10b6d871987a1b90..2b95955f444c5847d52f7ec59ded1b1e8c1c1181 100644
--- a/ui/aura/mus/window_manager_delegate.h
+++ b/ui/aura/mus/window_manager_delegate.h
@@ -148,6 +148,10 @@ class AURA_EXPORT WindowManagerDelegate {
       const gfx::Insets& insets,
       const std::vector<gfx::Rect>& additional_client_areas) = 0;
 
+  // Called when a client requests that its activation be given to another
+  // window.
+  virtual void OnWmDeactivateWindow(Window* window) = 0;
+
  protected:
   virtual ~WindowManagerDelegate() {}
 };