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

ash/test/ash_test_helper.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/test/ash_test_helper.h"
 
 #include "ash/accelerators/accelerator_controller.h"
 #include "ash/ash_switches.h"
 #include "ash/shell.h"
 #include "ash/test/display_manager_test_api.h"
@@ skipping 110 matching lines @@
   CHECK(!views::corewm::ScopedCaptureClient::IsActive());
 }
 
 void AshTestHelper::RunAllPendingInMessageLoop() {
   DCHECK(base::MessageLoopForUI::current() == message_loop_);
   aura::Env::CreateInstance();
   base::RunLoop run_loop(aura::Env::GetInstance()->GetDispatcher());
   run_loop.RunUntilIdle();
 }
 
-aura::RootWindow* AshTestHelper::CurrentContext() {
-  aura::RootWindow* root_window = Shell::GetTargetRootWindow();
+aura::Window* AshTestHelper::CurrentContext() {
+  aura::Window* root_window = Shell::GetTargetRootWindow();
   if (!root_window)
     root_window = Shell::GetPrimaryRootWindow();
   DCHECK(root_window);
   return root_window;
 }
 
 }  // namespace test
 }  // namespace ash