mus: Changes SetDisplayRoot() to create actual display

services/ui/display/screen_manager_ozone_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_DISPLAY_SCREEN_MANAGER_OZONE_EXTERNAL_H_
 #define SERVICES_UI_DISPLAY_SCREEN_MANAGER_OZONE_EXTERNAL_H_
 
 #include "services/ui/display/screen_manager.h"
 
 namespace display {
 
 // In external window mode, the purpose of having a ScreenManager
 // does not apply: there is not a ScreenManagerDelegate manager
 // responsible for creating Display instances.
 // Basically, in this mode WindowTreeHost creates the display instance.
 //
 // ScreenManagerOzoneExternal provides the stub out implementation
 // of ScreenManager for Ozone non-chromeos platforms.
 class ScreenManagerOzoneExternal : public ScreenManager {
  public:
   ScreenManagerOzoneExternal();
   ~ScreenManagerOzoneExternal() override;
 
  private:
   // ScreenManager.
   void AddInterfaces(service_manager::BinderRegistry* registry) override;
   void Init(ScreenManagerDelegate* delegate) override;
   void RequestCloseDisplay(int64_t display_id) override;
+  display::ScreenBase* GetScreen() override;
+
+  std::unique_ptr<display::ScreenBase> screen_;
 
   DISALLOW_COPY_AND_ASSIGN(ScreenManagerOzoneExternal);
 };
 
 }  // namespace display
 
 #endif  // SERVICES_UI_DISPLAY_SCREEN_MANAGER_OZONE_EXTERNAL_H_