Summary:
**Motivation**
I'm working on a project that uses React Native and needs to add direct synchronous bindings to native stuff through the JavaScriptCore C API. This is because it's performance-sensitive and would benefit from the quickest JS->C path. It does this using cross-platform C++ code that works on both iOS and Android. Most of the infrastructure for getting access to the JSC context is already in React Native actually, just had to add a few more things.
(lexs you mentioned to tag you in this pull request)
**Test plan**
Modify the JavaScriptCore context through the `JSContextRef` returned (eg. add an object at global scope) and verify that it exists in JavaScript.
Closes https://github.com/facebook/react-native/pull/10399
Differential Revision: D4080945
Pulled By: lexs
fbshipit-source-id: 6659b7a01e09fd84475adde183c1d3aca2d4cf09
Nikhilesh Sigatapu
Marc Horowitz
Michał Gregorczyk
David Aurelio
Alexander Blom
Chris Hopman