Hide Night Light behind a flag

ash/system/night_light/night_light_controller_unittest.cc

Index: ash/system/night_light/night_light_controller_unittest.cc
diff --git a/ash/system/night_light/night_light_controller_unittest.cc b/ash/system/night_light/night_light_controller_unittest.cc
index 175a01dcc88322f1c384b37df5b7323aafd23943..d0c48df67926aa3d6c168088a6ba12a98150bccd 100644
--- a/ash/system/night_light/night_light_controller_unittest.cc
+++ b/ash/system/night_light/night_light_controller_unittest.cc
@@ -76,6 +76,9 @@ class NightLightTest : public test::AshTestBase {
 
   // ash::test::AshTestBase:
   void SetUp() override {
+    // Explicitly enable the NightLight feature for the tests.
+    NightLightController::EnableFeatureForTesting();
+
     test::AshTestBase::SetUp();
     CreateTestUserSessions();
     Shell::RegisterPrefs(user1_pref_service_.registry());