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

ash/shelf/shelf_layout_manager_observer.h

Index: ash/shelf/shelf_layout_manager_observer.h
diff --git a/ash/shelf/shelf_layout_manager_observer.h b/ash/shelf/shelf_layout_manager_observer.h
index 328e2095d486c156e5ed6009a1c7864f51ccf3f5..6209980e15fece324009df80480b9a6269e6f091 100644
--- a/ash/shelf/shelf_layout_manager_observer.h
+++ b/ash/shelf/shelf_layout_manager_observer.h
@@ -9,7 +9,7 @@
 #include "ash/shelf/shelf_types.h"
 
 namespace aura {
-class RootWindow;
+class Window;
 }
 
 namespace ash {
@@ -28,7 +28,7 @@ class ASH_EXPORT ShelfLayoutManagerObserver {
   virtual void OnAutoHideStateChanged(ShelfAutoHideState new_state) {}
 
   // Called when the auto hide behavior is changed.
-  virtual void OnAutoHideBehaviorChanged(aura::RootWindow* root_window,
+  virtual void OnAutoHideBehaviorChanged(aura::Window* root_window,
                                          ShelfAutoHideBehavior new_behavior) {}
 };