mus: Changes SetDisplayRoot() to create actual display

services/ui/display/screen_manager.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_DISPLAY_SCREEN_MANAGER_H_
 #define SERVICES_UI_DISPLAY_SCREEN_MANAGER_H_
 
 #include <memory>
 
 #include "base/macros.h"
 #include "services/ui/display/screen_manager_delegate.h"
 
 namespace service_manager {
 class BinderRegistry;
 }
 
 namespace display {
 
+class ScreenBase;
+
 // ScreenManager provides the necessary functionality to configure all
 // attached physical displays.
 class ScreenManager {
  public:
   ScreenManager();
   virtual ~ScreenManager();
 
   // Creates a singleton ScreenManager instance.
   static std::unique_ptr<ScreenManager> Create();
   static ScreenManager* GetInstance();
 
   // Registers Mojo interfaces provided.
   virtual void AddInterfaces(service_manager::BinderRegistry* registry) = 0;
 
   // Triggers initial display configuration to start. On device this will
   // configuration the connected displays. Off device this will create one or
   // more fake displays and pretend to configure them. A non-null |delegate|
   // must be provided that will receive notifications when displays are added,
   // removed or modified.
   virtual void Init(ScreenManagerDelegate* delegate) = 0;
 
   // Handle requests from the platform to close a display.
   virtual void RequestCloseDisplay(int64_t display_id) = 0;
 
+  virtual display::ScreenBase* GetScreen() = 0;
+
  private:
   static ScreenManager* instance_;  // Instance is not owned.
 
   DISALLOW_COPY_AND_ASSIGN(ScreenManager);
 };
 
 }  // namespace display
 
 #endif  // SERVICES_UI_DISPLAY_SCREEN_MANAGER_H_