dillon1000/react

Unify perform{Sync,Concurrent}WorkOnRoot implementation (#31029)

Over time the behavior of these two paths has converged to be
essentially the same. So this merges them back into one function. This
should save some code size and also make it harder for the behavior to
accidentally diverge. (For the same reason, rolling out this change
might expose some areas where we had already accidentally diverged.)