Specify a default display UI scale to reset the zoom to

ash/display/display_manager.h

Index: ash/display/display_manager.h
diff --git a/ash/display/display_manager.h b/ash/display/display_manager.h
index 60c84089d7de2ef88d25b10656d52f090475b5c1..ac41d46cee3bbb14221440978168c40383e8cad1 100644
--- a/ash/display/display_manager.h
+++ b/ash/display/display_manager.h
@@ -162,9 +162,9 @@ class ASH_EXPORT DisplayManager
                           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