Introduce user customization of external HighDPI mode for 4K monitor.

chrome/browser/ui/webui/options/chromeos/display_options_handler.h

 // Copyright (c) 2012 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 CHROME_BROWSER_UI_WEBUI_OPTIONS_CHROMEOS_DISPLAY_OPTIONS_HANDLER_H_
 #define CHROME_BROWSER_UI_WEBUI_OPTIONS_CHROMEOS_DISPLAY_OPTIONS_HANDLER_H_
 
 #include <vector>
 
 #include "ash/display/display_controller.h"
@@ skipping 42 matching lines @@
   // left/top edge of the primary display.
   void OnFadeOutForDisplayLayoutFinished(int layout, int offset);
 
   // Handlers of JS messages.
   void HandleDisplayInfo(const base::ListValue* unused_args);
   void HandleMirroring(const base::ListValue* args);
   void HandleSetPrimary(const base::ListValue* args);
   void HandleDisplayLayout(const base::ListValue* args);
   void HandleSetUIScale(const base::ListValue* args);
   void HandleSetResolution(const base::ListValue* args);
+  void HandleSetDeviceScaleFactor(const base::ListValue* args);
   void HandleSetOrientation(const base::ListValue* args);
   void HandleSetColorProfile(const base::ListValue* args);
 
   DISALLOW_COPY_AND_ASSIGN(DisplayOptionsHandler);
 };
 
 }  // namespace options
 }  // namespace chromeos
 
 #endif  // CHROME_BROWSER_UI_WEBUI_OPTIONS_CHROMEOS_DISPLAY_OPTIONS_HANDLER_H_