Summary:
In some cases we need a way to specify some environmental data to shadow view
to improve layout (or do something similar), so `localData` serves these needs.
For example, any stateful embedded native views may benefit from this.
Have in mind that this data is not supposed to interfere with the state of
the shadow view.
Reviewed By: mmmulani
Differential Revision: D5884711
fbshipit-source-id: f0bf66e4608894ec4479b8aca262afcfba6b9f4b