window-actor: Special-case COMBO window's shadow class

We currently don't have any shadow class for combo box popups,
which means the default shadow of normal windows is used. That's
clearly odd given that the two are very different, and isn't
consistent with GTK+-3's client-side shadows for popups. While
we could add a dedicated shadow class, the designers are fine
with reusing the existing shadow for dropdown-menus, so let's
do that.

https://bugzilla.gnome.org/show_bug.cgi?id=744667

src/compositor/meta-window-actor.c

@@ -593,6 +593,7 @@ meta_window_actor_get_shadow_class (MetaWindowActor *self)
       switch (window_type)
         {
         case META_WINDOW_DROPDOWN_MENU:
+        case META_WINDOW_COMBO:
           return "dropdown-menu";
         case META_WINDOW_POPUP_MENU:
           return "popup-menu";