Hide Night Light behind a flag

ash/system/night_light/night_light_controller.h

Index: ash/system/night_light/night_light_controller.h
diff --git a/ash/system/night_light/night_light_controller.h b/ash/system/night_light/night_light_controller.h
index 8066d7c1ca45f55ac4a8e1353a0f54d89959a97d..bdef68d407bf91d9935b6562a33318449b371332 100644
--- a/ash/system/night_light/night_light_controller.h
+++ b/ash/system/night_light/night_light_controller.h
@@ -35,6 +35,9 @@ class ASH_EXPORT NightLightController : public SessionObserver {
   explicit NightLightController(SessionController* session_controller);
   ~NightLightController() override;
 
+  // Returns true if the NightLight feature is enabled in the flags.
+  static bool IsFeatureEnabled();
+
   static void RegisterPrefs(PrefRegistrySimple* registry);
 
   void AddObserver(Observer* observer);