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..f93dc0507899391c6906d12741d63ef3e7977372 100644
--- a/ash/display/display_manager.h
+++ b/ash/display/display_manager.h
@@ -166,6 +166,10 @@ class ASH_EXPORT DisplayManager
   // 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