Hide Night Light behind a flag

ash/shell.h

Index: ash/shell.h
diff --git a/ash/shell.h b/ash/shell.h
index f4aa13a9d688aa1a1955cf3004a27b9164b5c23e..4d5e9c36b887045b055e884de0b9bfeed0edcbad 100644
--- a/ash/shell.h
+++ b/ash/shell.h
@@ -337,9 +337,7 @@ class ASH_EXPORT Shell : public SessionObserver,
   NewWindowController* new_window_controller() {
     return new_window_controller_.get();
   }
-  NightLightController* night_light_controller() {
-    return night_light_controller_.get();
-  }
+  NightLightController* night_light_controller();
   SessionController* session_controller() { return session_controller_.get(); }
   ShelfController* shelf_controller() { return shelf_controller_.get(); }
   ShelfModel* shelf_model();