Summary: The Obj-C API is usually easier to work with, and also makes it very easy to use the C API when necessary (performance, for example). This diff just switches the designated initializer of RCTContextExecutor to take a JSContext instead of JSGlobalContextRef, but the old initializer still works if needed.
I was doing some memory leak investigation and it is easier with ARC so I wanted to incrementally move the executor to Obj-C.
Closes https://github.com/facebook/react-native/pull/2159
Reviewed By: svcscm
Differential Revision: D2554890
Pulled By: tadeuzagallo
fb-gh-sync-id: 75b96d04cddff68fa3daf5d0fafdffad21dae307
James Ide
Tadeu Zagallo
Jakub Zika