Specify a default display UI scale to reset the zoom to

ash/display/display_manager.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 ASH_DISPLAY_DISPLAY_MANAGER_H_
 #define ASH_DISPLAY_DISPLAY_MANAGER_H_
 
 #include <stddef.h>
 #include <stdint.h>
 
@@ skipping 148 matching lines @@
   // Sets the display's rotation for the given |source|. The new |rotation| will
   // also become active.
   void SetDisplayRotation(int64_t display_id,
                           display::Display::Rotation rotation,
                           display::Display::RotationSource source);
 
   // Sets the UI scale for the |display_id|. Returns false if the
   // display_id is not an internal display.
   bool SetDisplayUIScale(int64_t display_id, float scale);
 
+  // Resets the UI scale of the display with |display_id| to the one defined in
+  // the default mode.
+  bool ResetDisplayToDefaultMode(int64_t display_id);
+
   // Sets the external display's configuration, including resolution change,
   // ui-scale change, and device scale factor change. Returns true if it changes
   // the display resolution so that the caller needs to show a notification in
   // case the new resolution actually doesn't work.
   bool SetDisplayMode(
       int64_t display_id,
       const scoped_refptr<display::ManagedDisplayMode>& display_mode);
 
   // Register per display properties. |overscan_insets| is NULL if
   // the display has no custom overscan insets.
@@ skipping 298 matching lines @@
   base::ObserverList<display::DisplayObserver> observers_;
 
   base::WeakPtrFactory<DisplayManager> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(DisplayManager);
 };
 
 }  // namespace ash
 
 #endif  // ASH_DISPLAY_DISPLAY_MANAGER_H_