Hide Night Light behind a flag

ash/system/night_light/night_light_controller.cc

Index: ash/system/night_light/night_light_controller.cc
diff --git a/ash/system/night_light/night_light_controller.cc b/ash/system/night_light/night_light_controller.cc
index 90e3179f6d5a571c3caa709199c3be3db226fd29..cdbc238ad74baae8d1e5cd152f2ad9e1df656668 100644
--- a/ash/system/night_light/night_light_controller.cc
+++ b/ash/system/night_light/night_light_controller.cc
@@ -4,9 +4,11 @@
 
 #include "ash/system/night_light/night_light_controller.h"
 
+#include "ash/ash_switches.h"
 #include "ash/public/cpp/ash_pref_names.h"
 #include "ash/session/session_controller.h"
 #include "ash/shell.h"
+#include "base/command_line.h"
 #include "base/time/time.h"
 #include "components/prefs/pref_registry_simple.h"
 #include "components/prefs/pref_service.h"
@@ -56,6 +58,12 @@ NightLightController::~NightLightController() {
   session_controller_->RemoveObserver(this);
 }
 
+// static
+bool NightLightController::IsFeatureEnabled() {
+  return base::CommandLine::ForCurrentProcess()->HasSwitch(
+      ash::switches::kAshEnableNightLight);
+}
+
 // static
 void NightLightController::RegisterPrefs(PrefRegistrySimple* registry) {
   registry->RegisterBooleanPref(prefs::kNightLightEnabled, false);