Hide Night Light behind a flag

ash/shell.h

Index: ash/shell.h
diff --git a/ash/shell.h b/ash/shell.h
index f4aa13a9d688aa1a1955cf3004a27b9164b5c23e..26646cfd124e2a834ddd482cff8c07c1c5a59abd 100644
--- a/ash/shell.h
+++ b/ash/shell.h
@@ -13,6 +13,7 @@
 #include "ash/metrics/user_metrics_recorder.h"
 #include "ash/public/cpp/shelf_types.h"
 #include "ash/session/session_observer.h"
+#include "ash/system/night_light/night_light_controller.h"
 #include "ash/wm/system_modal_container_event_filter_delegate.h"
 #include "base/gtest_prod_util.h"
 #include "base/macros.h"
@@ -124,7 +125,6 @@ class MediaController;
 class MouseCursorEventFilter;
 class MruWindowTracker;
 class NewWindowController;
-class NightLightController;
 class OverlayEventFilter;
 class PaletteDelegate;
 class PartialMagnificationController;
@@ -338,6 +338,7 @@ class ASH_EXPORT Shell : public SessionObserver,
     return new_window_controller_.get();
   }
   NightLightController* night_light_controller() {
+    DCHECK(NightLightController::IsFeatureEnabled());

[James Cook, 2017/06/02 23:40:05]

If you want to do this I would move the implementation into the .cc file and not
include the header. (It's OK to have hacker_style() methods with the impl in the
.cc as long as the impl is cheap.)

[afakhry, 2017/06/03 00:09:02]

Done.

     return night_light_controller_.get();
   }
   SessionController* session_controller() { return session_controller_.get(); }