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

ash/shell_observer.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_SHELL_OBSERVER_H_
 #define ASH_SHELL_OBSERVER_H_
 
 #include "ash/ash_export.h"
 #include "ash/system/user/login_status.h"
 
 namespace aura {
-class RootWindow;
+class Window;
 }
 
 namespace ash {
 
 class ASH_EXPORT ShellObserver {
  public:
   // Invoked after the screen's work area insets changes.
   virtual void OnDisplayWorkAreaInsetsChanged() {}
 
   // Invoked when the user logs in.
   virtual void OnLoginStateChanged(user::LoginStatus status) {}
 
   // Invoked when the application is exiting.
   virtual void OnAppTerminating() {}
 
   // Invoked when the screen is locked (after the lock window is visible) or
   // unlocked.
   virtual void OnLockStateChanged(bool locked) {}
 
   // Invoked when the shelf alignment in |root_window| is changed.
-  virtual void OnShelfAlignmentChanged(aura::RootWindow* root_window) {}
+  virtual void OnShelfAlignmentChanged(aura::Window* root_window) {}
 
   // Invoked when the projection touch HUD is toggled.
   virtual void OnTouchHudProjectionToggled(bool enabled) {}
 
   // Invoked when entering or exiting fullscreen mode in |root_window|.
   virtual void OnFullscreenStateChanged(bool is_fullscreen,
-                                        aura::RootWindow* root_window) {}
+                                        aura::Window* root_window) {}
 
  protected:
   virtual ~ShellObserver() {}
 };
 
 }  // namespace ash
 
 #endif  // ASH_SHELL_OBSERVER_H_