Makes SetFocus() notify callback correctly on failure

components/mus/ws/window_server.h

Index: components/mus/ws/window_server.h
diff --git a/components/mus/ws/window_server.h b/components/mus/ws/window_server.h
index b05efb8e1369c51d341176e5b51d50648e63d7de..cf945a0d00d5257dec3b55dc202382300804a6f9 100644
--- a/components/mus/ws/window_server.h
+++ b/components/mus/ws/window_server.h
@@ -134,7 +134,10 @@ class WindowServer : public ServerWindowDelegate,
     return &window_manager_factory_registry_;
   }
 
-  void SetFocusedWindow(ServerWindow* window);
+  // Sets focus to |window|. Returns true if |window| already has focus, or
+  // focus was successfully changed. Returns |false| if |window| is not a valid
+  // window to receive focus.
+  bool SetFocusedWindow(ServerWindow* window);
   ServerWindow* GetFocusedWindow();
 
   // Returns a change id for the window manager that is associated with