Summary: We have some "internal-only" stuff we want to put in the attribute of a layout node. This allows us to do so a way that keeps it out of open-source and avoid the clunky subclassing. Reviewed By: adamjernst Differential Revision: D24162047 fbshipit-source-id: 518ab9fbf0b4274076f5b0ae1b249e197563fa7f