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

ui/aura/client/window_tree_client.cc

 // Copyright (c) 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 "ui/aura/client/window_tree_client.h"
 
 #include "ui/aura/env.h"
 #include "ui/aura/root_window.h"
 #include "ui/aura/window_property.h"
 
 DECLARE_WINDOW_PROPERTY_TYPE(aura::client::WindowTreeClient*)
 
 namespace aura {
 namespace client {
 
 DEFINE_WINDOW_PROPERTY_KEY(
     WindowTreeClient*, kRootWindowWindowTreeClientKey, NULL);
 
 void SetWindowTreeClient(Window* window, WindowTreeClient* window_tree_client) {
   DCHECK(window);
 
-  RootWindow* root_window = window->GetRootWindow();
+  Window* root_window = window->GetRootWindow();
   DCHECK(root_window);
   root_window->SetProperty(kRootWindowWindowTreeClientKey, window_tree_client);
 }
 
 WindowTreeClient* GetWindowTreeClient(Window* window) {
   DCHECK(window);
-  RootWindow* root_window = window->GetRootWindow();
+  Window* root_window = window->GetRootWindow();
   DCHECK(root_window);
   WindowTreeClient* client =
       root_window->GetProperty(kRootWindowWindowTreeClientKey);
   DCHECK(client);
   return client;
 }
 
 void ParentWindowWithContext(Window* window,
                              Window* context,
                              const gfx::Rect& screen_bounds) {
   DCHECK(context);
 
   // |context| must be attached to a hierarchy with a WindowTreeClient.
   WindowTreeClient* client = GetWindowTreeClient(context);
   DCHECK(client);
   Window* default_parent =
       client->GetDefaultParent(context, window, screen_bounds);
   default_parent->AddChild(window);
 }
 
 }  // namespace client
 }  // namespace aura