It'll be moved to the role owning it, accessed via a helper function implemented by the role. Currently it still just fetches the field in MetaWaylandSurface. https://gitlab.gnome.org/GNOME/mutter/merge_requests/835
Does some needed naming cleanup while at it, to be more similar to other role types. In short, MetaWaylandSurfaceRoleXwayland was changed to MetaXwaylandSurface. https://gitlab.gnome.org/GNOME/mutter/merge_requests/835