Hide Night Light behind a flag

ash/system/tiles/tray_tiles_unittest.cc

Index: ash/system/tiles/tray_tiles_unittest.cc
diff --git a/ash/system/tiles/tray_tiles_unittest.cc b/ash/system/tiles/tray_tiles_unittest.cc
index 7341f72566050af1302add213ae340d0d5eb6e0c..30f6cf9e00efc588f9c745703261fd7fce464d33 100644
--- a/ash/system/tiles/tray_tiles_unittest.cc
+++ b/ash/system/tiles/tray_tiles_unittest.cc
@@ -4,6 +4,7 @@
 
 #include "ash/system/tiles/tray_tiles.h"
 
+#include "ash/system/night_light/night_light_controller.h"
 #include "ash/system/night_light/night_light_toggle_button.h"
 #include "ash/system/tiles/tiles_default_view.h"
 #include "ash/system/tray/system_menu_button.h"
@@ -23,6 +24,7 @@ class TrayTilesTest : public test::NoSessionAshTestBase {
   ~TrayTilesTest() override {}
 
   void SetUp() override {
+    NightLightController::EnableFeatureForTesting();
     test::NoSessionAshTestBase::SetUp();
     tray_tiles_.reset(new TrayTiles(GetPrimarySystemTray()));
   }