Use correct theme for the off icon of NightLight, fix tooltip and a11y

ash/system/tiles/tiles_default_view.h

Index: ash/system/tiles/tiles_default_view.h
diff --git a/ash/system/tiles/tiles_default_view.h b/ash/system/tiles/tiles_default_view.h
index 41ddbef5599068a2945fbbd8c159e75aaa4aa44a..44d6987d75a61d80f025cc7b8a4acb4b89eb6411 100644
--- a/ash/system/tiles/tiles_default_view.h
+++ b/ash/system/tiles/tiles_default_view.h
@@ -15,7 +15,7 @@ class CustomButton;
 }
 
 namespace ash {
-class SystemMenuButton;
+class NightLightToggleButton;
 class SystemTrayItem;
 
 // The container view for the tiles in the bottom row of the system menu
@@ -49,7 +49,7 @@ class ASH_EXPORT TilesDefaultView : public views::View,
   // which case the corresponding pointer will be null.
   views::CustomButton* settings_button_;
   views::CustomButton* help_button_;
-  SystemMenuButton* night_light_button_;
+  NightLightToggleButton* night_light_button_;
   views::CustomButton* lock_button_;
   views::CustomButton* power_button_;