dillon1000/react

Track which fibers scheduled the current render work (#15658)

Tracked Fibers are called "updaters" and are exposed to DevTools via a 'memoizedUpdaters' property on the ReactFiberRoot. The implementation of this feature follows a vaguely similar approach as interaction tracing, but does not require reference counting since there is no subscriptions API.

This change is in support of a new DevTools Profiler feature that shows which Fiber(s) scheduled the selected commit in the Profiler.

All changes have been gated behind a new feature flag, 'enableUpdaterTracking', which is enabled for Profiling builds by default. We also only track updaters when DevTools has been detected, to avoid doing unnecessary work.