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

ash/scoped_target_root_window.cc

 // Copyright 2013 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.
 #include "ash/scoped_target_root_window.h"
 
 #include "ash/shell.h"
 
 namespace ash {
 namespace internal {
 
 ScopedTargetRootWindow::ScopedTargetRootWindow(
-    aura::RootWindow* root_window) {
+    aura::Window* root_window) {
   Shell::GetInstance()->scoped_target_root_window_ = root_window;
 }
 
 ScopedTargetRootWindow::~ScopedTargetRootWindow() {
   Shell::GetInstance()->scoped_target_root_window_ = NULL;
 }
 
 }  // namespace internal
 }  // namespace ash