Summary:
Changelog: [internal]
Introduces synchronous access to the runtime from RuntimeScheduler.
At the moment, this is not used anywhere.
In case RuntimeScheduler isn't defined (controlled by MC), falls back to RuntimeExecutor.
Reviewed By: mdvacca
Differential Revision: D28024380
fbshipit-source-id: 90be36dd390202540ed51940a4396040f043cd90