Summary:
`RunLoopObserver` is one of the core interfaces that bridge intrinsically platform-specific functionality to cross-platform React Native core. `RunLoopObserver` allows subscribing for notifications about changes in a run loop life cycle. Primarily it supposed to be used for observing UI (aka main) and JavaScript execution thread/run-loop.
Having a `RunLoopObserver` implemented in a platform-specific manner allows building these components in a cross-platform manner:
* Sync and async UI event delivery pipeline;
* Timing for some animation engine;
* Timers (probably additional features are required).
Changelog: [Internal] Fabric-specific internal change.
Reviewed By: sammy-SC
Differential Revision: D21341997
fbshipit-source-id: 7ef61fb51f550dd0f2e89c64af657e0f0de029aa
Valentin Shergin