Make GetRootWindow() return a Window instead of a RootWindow.

ash/display/display_controller.h

Index: ash/display/display_controller.h
diff --git a/ash/display/display_controller.h b/ash/display/display_controller.h
index dbe5f508bed8c847ce776e1c619e09491fc674fa..01c6932896d88609d7b714d8829f28a0cd5cf538 100644
--- a/ash/display/display_controller.h
+++ b/ash/display/display_controller.h
@@ -93,10 +93,10 @@ class ASH_EXPORT DisplayController : public gfx::DisplayObserver,
   void RemoveObserver(Observer* observer);
 
   // Returns the root window for primary display.
-  aura::RootWindow* GetPrimaryRootWindow();
+  aura::Window* GetPrimaryRootWindow();
 
   // Returns the root window for |display_id|.
-  aura::RootWindow* GetRootWindowForDisplayId(int64 id);
+  aura::Window* GetRootWindowForDisplayId(int64 id);
 
   // Toggle mirror mode.
   void ToggleMirrorMode();