Hide Night Light behind a flag

ash/system/night_light/night_light_controller.h

 // Copyright 2017 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef ASH_SYSTEM_NIGHT_LIGHT_NIGHT_LIGHT_CONTROLLER_H_
 #define ASH_SYSTEM_NIGHT_LIGHT_NIGHT_LIGHT_CONTROLLER_H_
 
 #include <memory>
 
 #include "ash/ash_export.h"
@@ skipping 17 matching lines @@
     // Emitted when the NightLight status is changed.
     virtual void OnNightLightEnabledChanged(bool enabled) = 0;
 
    protected:
     virtual ~Observer() {}
   };
 
   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);
   void RemoveObserver(Observer* observer);
 
   // Get the NightLight settings stored in the current active user prefs.
   bool GetEnabled() const;
   float GetColorTemperature() const;
 
   // Set the desired NightLight settings in the current active user prefs.
@@ skipping 35 matching lines @@
   std::unique_ptr<PrefChangeRegistrar> pref_change_registrar_;
 
   base::ObserverList<Observer> observers_;
 
   DISALLOW_COPY_AND_ASSIGN(NightLightController);
 };
 
 }  // namespace ash
 
 #endif  // ASH_SYSTEM_NIGHT_LIGHT_NIGHT_LIGHT_CONTROLLER_H_