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

ash/wm/coordinate_conversion.cc

 // 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.
 
 #include "ash/wm/coordinate_conversion.h"
 
 #include "ash/display/display_controller.h"
 #include "ash/shell.h"
 #include "ui/aura/client/screen_position_client.h"
-#include "ui/aura/root_window.h"
 #include "ui/gfx/display.h"
 #include "ui/gfx/point.h"
 #include "ui/gfx/rect.h"
 #include "ui/gfx/screen.h"
 
 namespace ash {
 namespace wm {
 
-aura::RootWindow* GetRootWindowAt(const gfx::Point& point) {
+aura::Window* GetRootWindowAt(const gfx::Point& point) {
   const gfx::Display& display =
       Shell::GetScreen()->GetDisplayNearestPoint(point);
   DCHECK(display.is_valid());
   // TODO(yusukes): Move coordinate_conversion.cc and .h to ui/aura/ once
   // GetRootWindowForDisplayId() is moved to aura::Env.
   return Shell::GetInstance()->display_controller()->
       GetRootWindowForDisplayId(display.id());
 }
 
-aura::RootWindow* GetRootWindowMatching(const gfx::Rect& rect) {
+aura::Window* GetRootWindowMatching(const gfx::Rect& rect) {
   const gfx::Display& display = Shell::GetScreen()->GetDisplayMatching(rect);
   return Shell::GetInstance()->display_controller()->
       GetRootWindowForDisplayId(display.id());
 }
 
 void ConvertPointToScreen(aura::Window* window, gfx::Point* point) {
   aura::client::GetScreenPositionClient(window->GetRootWindow())->
       ConvertPointToScreen(window, point);
 }
 
 void ConvertPointFromScreen(aura::Window* window,
                             gfx::Point* point_in_screen) {
   aura::client::GetScreenPositionClient(window->GetRootWindow())->
       ConvertPointFromScreen(window, point_in_screen);
 }
 
 }  // namespace wm
 }  // namespace ash