Summary:
JS API very similar to web workers and node's child process.
Work has been done by somebody else for the Android implementation over at #7020, so we'd need to have these in sync before anything gets merged.
I've made a prop `messagingEnabled` to be more explicit about creating globals—it might be sufficient to just check for an onMessage handler though.
Closes https://github.com/facebook/react-native/pull/9762
Differential Revision: D4008260
fbshipit-source-id: 84b1afafbc0ab1edc3dfbf1a8fb870218e171a4c
