Hide Night Light behind a flag

ash/system/tray/system_tray.cc

Index: ash/system/tray/system_tray.cc
diff --git a/ash/system/tray/system_tray.cc b/ash/system/tray/system_tray.cc
index 75c9c119fb2586f0e4369cd45aea67628fd445ee..3b098244e737ab33ddb5d42fbac4bf52d6cf693c 100644
--- a/ash/system/tray/system_tray.cc
+++ b/ash/system/tray/system_tray.cc
@@ -265,8 +265,10 @@ void SystemTray::CreateItems(SystemTrayDelegate* delegate) {
   AddTrayItem(base::WrapUnique(tray_scale_));
   AddTrayItem(base::MakeUnique<TrayBrightness>(this));
   AddTrayItem(base::MakeUnique<TrayCapsLock>(this));
-  tray_night_light_ = new TrayNightLight(this);
-  AddTrayItem(base::WrapUnique(tray_night_light_));
+  if (NightLightController::IsFeatureEnabled()) {
+    tray_night_light_ = new TrayNightLight(this);
+    AddTrayItem(base::WrapUnique(tray_night_light_));
+  }
   // TODO(jamescook): Remove this when mash has support for display management
   // and we have a DisplayManager equivalent. See http://crbug.com/548429
   if (Shell::GetAshConfig() != Config::MASH)