Extract shell methods from ApplicationImpl into a base class, and pass this to Initialize() instead.

components/mus/public/cpp/lib/window_tree_host_factory.cc

 // Copyright 2015 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 "components/mus/public/cpp/window_tree_host_factory.h"
 
 #include "components/mus/public/cpp/window_tree_connection.h"
 #include "components/mus/public/cpp/window_tree_delegate.h"
-#include "mojo/shell/public/cpp/application_impl.h"
+#include "mojo/shell/public/cpp/shell.h"
 
 namespace mus {
 
 void CreateWindowTreeHost(mojom::WindowTreeHostFactory* factory,
                           WindowTreeDelegate* delegate,
                           mojom::WindowTreeHostPtr* host,
                           WindowManagerDelegate* window_manager_delegate) {
   mojom::WindowTreeClientPtr tree_client;
   WindowTreeConnection::CreateForWindowManager(
       delegate, GetProxy(&tree_client),
       WindowTreeConnection::CreateType::DONT_WAIT_FOR_EMBED,
       window_manager_delegate);
   factory->CreateWindowTreeHost(GetProxy(host), std::move(tree_client));
 }
 
-void CreateWindowTreeHost(mojo::ApplicationImpl* app,
+void CreateWindowTreeHost(mojo::Shell* shell,
                           WindowTreeDelegate* delegate,
                           mojom::WindowTreeHostPtr* host,
                           WindowManagerDelegate* window_manager_delegate) {
   mojom::WindowTreeHostFactoryPtr factory;
-  app->ConnectToService("mojo:mus", &factory);
+  shell->ConnectToService("mojo:mus", &factory);
   CreateWindowTreeHost(factory.get(), delegate, host, window_manager_delegate);
 }
 
 }  // namespace mus