Mus Demo: Refactor code to prepare multiple windows in external mode

services/ui/demo/mus_demo_external.h

 // Copyright 2017 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.
 
 #ifndef SERVICES_UI_DEMO_MUS_DEMO_EXTERNAL_H_
 #define SERVICES_UI_DEMO_MUS_DEMO_EXTERNAL_H_
 
 #include <memory>
 #include <vector>
 
 #include "services/ui/demo/mus_demo.h"
 #include "services/ui/public/interfaces/window_tree_host.mojom.h"
 
 namespace ui {
 namespace demo {
 
 // MusDemoExternal demonstrates Mus operating in "external" mode: A new platform
 // window (and hence acceleratedWidget) is created for each aura window.
 class MusDemoExternal : public MusDemo {
  public:
   MusDemoExternal();
   ~MusDemoExternal() final;
 
  private:
+  void OpenNewWindow();
+
   // ui::demo::MusDemo:
-  void OnStartImpl(std::unique_ptr<aura::WindowTreeClient>* window_tree_client,
-                   std::unique_ptr<WindowTreeData>* window_tree_data) final;
+  void OnStartImpl() final;
+  std::unique_ptr<aura::WindowTreeClient> CreateWindowTreeClient() final;
 
   // aura::WindowTreeClientDelegate:
   void OnEmbed(std::unique_ptr<aura::WindowTreeHostMus> window_tree_host) final;
   void OnEmbedRootDestroyed(aura::WindowTreeHostMus* window_tree_host) final;
 
   mojom::WindowTreeHostFactoryPtr window_tree_host_factory_;
+  mojom::WindowTreeClientPtr window_tree_client_mojo_;
+  size_t initialized_windows_count_ = 0;
 
   DISALLOW_COPY_AND_ASSIGN(MusDemoExternal);
 };
 
 }  // namespace demo
 }  // namespace ui
 
 #endif  // SERVICES_UI_DEMO_MUS_DEMO_EXTERNAL_H_