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

ui/aura/root_window.h

Index: ui/aura/root_window.h
diff --git a/ui/aura/root_window.h b/ui/aura/root_window.h
index ad4c3eb4ca6689fe601993872deb0ba129da17d3..c055b1a2d8407775bddd8825b48d377d3d9fc1bc 100644
--- a/ui/aura/root_window.h
+++ b/ui/aura/root_window.h
@@ -238,8 +238,8 @@ class AURA_EXPORT RootWindow : public Window,
   gfx::Transform GetRootTransform() const;
 
   // Overridden from Window:
-  virtual RootWindow* GetRootWindow() OVERRIDE;
-  virtual const RootWindow* GetRootWindow() const OVERRIDE;
+  virtual Window* GetRootWindow() OVERRIDE;
+  virtual const Window* GetRootWindow() const OVERRIDE;
   virtual void SetTransform(const gfx::Transform& transform) OVERRIDE;
   virtual bool CanFocus() const OVERRIDE;
   virtual bool CanReceiveEvents() const OVERRIDE;
@@ -281,7 +281,7 @@ class AURA_EXPORT RootWindow : public Window,
 
   // Called when a Window is attached or detached from the RootWindow.
   void OnWindowAddedToRootWindow(Window* window);
-  void OnWindowRemovedFromRootWindow(Window* window, RootWindow* new_root);
+  void OnWindowRemovedFromRootWindow(Window* window, Window* new_root);
 
   // Called when a window becomes invisible, either by being removed
   // from root window hierarchy, via SetVisible(false) or being destroyed.