Summary:
If tracking is enabled and the sampling check passes on a scroll or layout event,
we compare the scroll offset to the layout of the rendered items. If the items don't cover
the visible area of the list, we fire an `onFillRateExceeded` call with relevant stats for
logging the event through an analytics pipeline.
The measurement methodology is a little jank because everything is async, but it seems directionally
useful for getting ballpark numbers, catching regressions, and tracking improvements.
Benchmark testing shows a ~2014 MotoX starts hitting the fill rate limit at about 2500 px / sec,
which is pretty fast scrolling.
This also reworks our frame rate stuff so we can use a shared `SceneTracking` thing and track blankness
globally.
Reviewed By: bvaughn
Differential Revision: D4806867
fbshipit-source-id: 119bf177463c8c3aa51fa13d1a9d03b1a96042aa
Spencer Ahrens