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

ash/display/resolution_notification_controller.h

Index: ash/display/resolution_notification_controller.h
diff --git a/ash/display/resolution_notification_controller.h b/ash/display/resolution_notification_controller.h
index 9fe402650177143a4d426275679adaafadcb51ea..9d202b474b418386c9d953dbf1d40b29525fa903 100644
--- a/ash/display/resolution_notification_controller.h
+++ b/ash/display/resolution_notification_controller.h
@@ -24,6 +24,8 @@ class Widget;
 
 namespace ash {
 
+struct DisplayMode;
+
 // A class which manages the notification of display resolution change and
 // also manages the timeout in case the new resolution is unusable.
 class ASH_EXPORT ResolutionNotificationController
@@ -41,8 +43,8 @@ class ASH_EXPORT ResolutionNotificationController
   // changes if something goes wrong.
   void SetDisplayResolutionAndNotify(
       int64 display_id,
-      const gfx::Size& old_resolution,
-      const gfx::Size& new_resolution,
+      const DisplayMode& old_resolution,
+      const DisplayMode& new_resolution,
       const base::Closure& accept_callback);
 
   // Returns true if the notification is visible or scheduled to be visible and