mus: Fix ash::GetRootWindowController for child windows/widgets

ash/root_window_controller.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef ASH_ROOT_WINDOW_CONTROLLER_H_
 #define ASH_ROOT_WINDOW_CONTROLLER_H_
 
 #include <map>
 #include <memory>
 
@@ skipping 288 matching lines @@
   // Handles double clicks on the panel window header.
   std::unique_ptr<ui::EventHandler> panel_container_handler_;
 
   std::unique_ptr<DesktopBackgroundWidgetController> wallpaper_controller_;
   std::unique_ptr<AnimatingDesktopController> animating_wallpaper_controller_;
   std::unique_ptr<::wm::ScopedCaptureClient> capture_client_;
 
   DISALLOW_COPY_AND_ASSIGN(RootWindowController);
 };
 
-
-// Gets the RootWindowController for |root_window|.
+// On classic ash, returns the RootWindowController for the given |root_window|.
+// On mus ash, returns the RootWindowController for the primary display.
+// See RootWindowController class comment above.
 ASH_EXPORT RootWindowController* GetRootWindowController(
     const aura::Window* root_window);
 
 }  // namespace ash
 
 #endif  // ASH_ROOT_WINDOW_CONTROLLER_H_