Re-factor system tray code controlling launcher visibility

ash/system/tray/tray_background_view.h

Index: ash/system/tray/tray_background_view.h
diff --git a/ash/system/tray/tray_background_view.h b/ash/system/tray/tray_background_view.h
index 9864d7b17cd5b5695ed06f506e0ee521c7c7352b..61b3e42a9052a75fdb8d446589992bb66e3d28c2 100644
--- a/ash/system/tray/tray_background_view.h
+++ b/ash/system/tray/tray_background_view.h
@@ -90,6 +90,9 @@ class ASH_EXPORT TrayBackgroundView : public internal::ActionableView,
       bool value,
       internal::BackgroundAnimator::ChangeType change_type);
 
+  // Convenience function to call same function in status_area_widget_.
+  void UpdateShouldShowLauncher();
+
   StatusAreaWidget* status_area_widget() {
     return status_area_widget_;
   }