Hide Night Light behind a flag

ash/system/night_light/tray_night_light_unittest.cc

 // 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.
 
 #include "ash/system/night_light/tray_night_light.h"
 
 #include "ash/public/cpp/config.h"
 #include "ash/shell.h"
 #include "ash/system/night_light/night_light_controller.h"
 #include "ash/system/tray/system_tray.h"
 #include "ash/test/ash_test_base.h"
 #include "ash/test/ash_test_helper.h"
 #include "ash/test/test_shell_delegate.h"
 #include "base/macros.h"
 #include "components/prefs/testing_pref_service.h"
 
 namespace ash {
 
 namespace {
 
 constexpr char kFakeUserEmail[] = "fake_user@nightlight";
 
 class TrayNightLightTest : public test::AshTestBase {
  public:
   TrayNightLightTest() = default;
   ~TrayNightLightTest() override = default;
 
   // ash::test::AshTestBase:
   void SetUp() override {
+    NightLightController::EnableFeatureForTesting();
     test::AshTestBase::SetUp();
     GetSessionControllerClient()->Reset();
     GetSessionControllerClient()->AddUserSession(kFakeUserEmail);
     Shell::RegisterPrefs(pref_service_.registry());
 
     ash_test_helper()->test_shell_delegate()->set_active_user_pref_service(
         &pref_service_);
     GetSessionControllerClient()->SwitchActiveUser(
         AccountId::FromUserEmail(kFakeUserEmail));
   }
@@ skipping 23 matching lines @@
   ASSERT_FALSE(controller->GetEnabled());
   EXPECT_FALSE(tray_night_light->tray_view()->visible());
   controller->Toggle();
   ASSERT_TRUE(controller->GetEnabled());
   EXPECT_TRUE(tray_night_light->tray_view()->visible());
 }
 
 }  // namespace
 
 }  // namespace ash