Summary:
iOS follow up to #8569. This currently depends on the Android PR since it contains the JS implementation, only review the last commit. Just putting this out here for visibility, don't merge this before the Android PR.
**Test plan**
Tested by running a background task that burns all remaining idle time (see UIExplorer example).
Tested that native only calls into JS when there are pending idle callbacks.
Tested that timers are executed before idle callback.
Closes https://github.com/facebook/react-native/pull/8734
Differential Revision: D3560818
fbshipit-source-id: a28d3092377a7fd4331647148d40fe69e4198c7e
5618c3ff09