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..8307834d701a45d8d1ba339b5324ccaa481a8fd8 100644
--- a/ash/system/night_light/night_light_controller_unittest.cc
+++ b/ash/system/night_light/night_light_controller_unittest.cc
@@ -7,6 +7,7 @@
 #include <cmath>
 #include <limits>
 
+#include "ash/ash_switches.h"
 #include "ash/public/cpp/ash_pref_names.h"
 #include "ash/public/cpp/config.h"
 #include "ash/public/cpp/session_types.h"
@@ -15,6 +16,7 @@
 #include "ash/test/ash_test_helper.h"
 #include "ash/test/test_session_controller_client.h"
 #include "ash/test/test_shell_delegate.h"
+#include "base/command_line.h"
 #include "base/macros.h"
 #include "components/prefs/testing_pref_service.h"
 #include "ui/compositor/layer.h"
@@ -76,6 +78,10 @@ class NightLightTest : public test::AshTestBase {
 
   // ash::test::AshTestBase:
   void SetUp() override {
+    // Explicitly enable the NightLight feature for the tests.
+    base::CommandLine::ForCurrentProcess()->AppendSwitch(
+        ash::switches::kAshEnableNightLight);
+
     test::AshTestBase::SetUp();
     CreateTestUserSessions();
     Shell::RegisterPrefs(user1_pref_service_.registry());