Add mojom::DisplayController.

services/ui/display/platform_screen.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_PLATFORM_SCREEN_H_
 #define SERVICES_UI_DISPLAY_PLATFORM_SCREEN_H_
 
 #include <memory>
 
+#include "base/macros.h"
 #include "services/ui/display/platform_screen_delegate.h"
 
 namespace display {
 
 // PlatformScreen provides the necessary functionality to configure all
 // attached physical displays.
 class PlatformScreen {
  public:
-  virtual ~PlatformScreen() {}
+  PlatformScreen();
+  virtual ~PlatformScreen();
 
-  // Creates a PlatformScreen instance.
+  // Creates a singleton PlatformScreen instance.
   static std::unique_ptr<PlatformScreen> Create();
+  static PlatformScreen* GetInstance();
 
   // 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(PlatformScreenDelegate* delegate) = 0;
 
   virtual int64_t GetPrimaryDisplayId() const = 0;
+
+ private:
+  static PlatformScreen* instance_;  // Instance is not owned.
+
+  DISALLOW_COPY_AND_ASSIGN(PlatformScreen);
 };
 
 }  // namespace display
 
 #endif  // SERVICES_UI_DISPLAY_PLATFORM_SCREEN_H_