Start to decouple display::Display from ws::PlatformDisplay.

services/ui/ws/platform_display_delegate.h

 // Copyright 2015 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_WS_PLATFORM_DISPLAY_DELEGATE_H_
 #define SERVICES_UI_WS_PLATFORM_DISPLAY_DELEGATE_H_
 
 namespace display {
 class Display;
 }
 
 namespace ui {
 
 class Event;
 
 namespace ws {
 
 class ServerWindow;
 
 // PlatformDisplayDelegate is implemented by an object that manages the
 // lifetime of a PlatformDisplay, forwards events to the appropriate windows,
 /// and responses to changes in viewport size.
 class PlatformDisplayDelegate {
  public:
   // Returns a display::Display for this display.
-  virtual display::Display GetDisplay() = 0;
+  virtual const display::Display& GetDisplay() = 0;
 
   // Returns the root window of this display.
   virtual ServerWindow* GetRootWindow() = 0;
 
   // Called once when the AcceleratedWidget is available for drawing.
   virtual void OnAcceleratedWidgetAvailable() = 0;
 
   virtual bool IsInHighContrastMode() = 0;
 
   // Called when an event arrives.
   virtual void OnEvent(const ui::Event& event) = 0;
 
   // Called when the Display loses capture.
   virtual void OnNativeCaptureLost() = 0;
 
  protected:
   virtual ~PlatformDisplayDelegate() {}
 };
 
 }  // namespace ws
 
 }  // namespace ui
 
 #endif  // SERVICES_UI_WS_PLATFORM_DISPLAY_DELEGATE_H_