Reland Mus Demo: Move definition of WindowTreeData into separate files

services/ui/demo/mus_demo.h

 // Copyright 2016 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_H_
 #define SERVICES_UI_DEMO_MUS_DEMO_H_
 
 #include <map>
 #include <memory>
 #include <set>
@@ skipping 17 matching lines @@
 }
 }  // namespace aura
 
 namespace wm {
 class WMState;
 }
 
 namespace ui {
 namespace demo {
 
+class WindowTreeData;
+
 // A simple MUS Demo service. This service connects to the service:ui, adds a
 // new window to the root Window, and draws a spinning square in the center of
 // the window. Provides a simple way to demonstrate that the graphic stack works
 // as intended.
 class MusDemo : public service_manager::Service,
                 public aura::WindowTreeClientDelegate,
                 public aura::WindowManagerDelegate {
  public:
   MusDemo();
   ~MusDemo() override;
@@ skipping 47 matching lines @@
   void OnWmDeactivateWindow(aura::Window* window) override;
 
   std::unique_ptr<aura::WindowTreeClient> window_tree_client_;
   std::unique_ptr<aura::Env> env_;
   std::unique_ptr<display::ScreenBase> screen_;
 
   std::unique_ptr<aura::client::DefaultCaptureClient> capture_client_;
   std::unique_ptr<::wm::WMState> wm_state_;
   std::unique_ptr<aura::PropertyConverter> property_converter_;
 
-  class WindowTreeData;
   std::unique_ptr<WindowTreeData> window_tree_data_;
 
   DISALLOW_COPY_AND_ASSIGN(MusDemo);
 };
 
 }  // namespace demo
 }  // namespace aura
 
 #endif  // SERVICES_UI_DEMO_MUS_DEMO_H_