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..d2e38273b7554d9225c38a66d6205f5434a56d01 100644
--- a/ash/system/night_light/night_light_controller.h
+++ b/ash/system/night_light/night_light_controller.h
@@ -35,6 +35,13 @@ 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();
+
+  // Enables the NightLight feature by adding its corresponding flag for unit
+  // tests.
+  static void EnableFeatureForTesting();
+
   static void RegisterPrefs(PrefRegistrySimple* registry);
 
   void AddObserver(Observer* observer);