Specify a default display UI scale to reset the zoom to

ash/test/display_manager_test_api.h

Index: ash/test/display_manager_test_api.h
diff --git a/ash/test/display_manager_test_api.h b/ash/test/display_manager_test_api.h
index 5624aa82810d993990ea35163cb5d6fda18c7012..0daff0327919eb462e7c02bd0c7cca98d75c26ac 100644
--- a/ash/test/display_manager_test_api.h
+++ b/ash/test/display_manager_test_api.h
@@ -62,6 +62,10 @@ class DisplayManagerTestApi {
   const display::ManagedDisplayInfo& GetInternalManagedDisplayInfo(
       int64_t display_id);
 
+  // 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);
+
  private:
   friend class ScopedSetInternalDisplayId;
   // Sets the display id for internal display and