dillon1000/react

Bugfix: Don't rely on `finishedLanes` for passive effects (#21233)

I recently started using `pendingPassiveEffectsLanes` to check if there were any pending
passive effects (530027a). `pendingPassiveEffectsLanes` is the value of
`root.finishedLanes` at the beginning of the commit phase. When there
are pending passive effects, it should always be a non-zero value,
because it represents the lanes used to render the effects.

But it turns out that `root.finishedLanes` isn't always correct.
Sometimes it's `NoLanes` even when there's a new commit.

I found this while investigating an internal bug report. The only repro
I could get was via a headless e2e test runner; I couldn't get one in an
actual browser, or other interactive environment. I used the e2e test to
bisect and confirm the fix. But I don't know yet know how to write a
regression test for the precise underlying scenario. I can probably
reverse engineer one by studying the code; after a quick glance
at `performConcurrentWorkOnRoot` and `performSyncWorkOnRoot`, it's not
hard to see how this might happen.

In the meantime, I'll revert the recent change that exposed the bug.

I was surprised that this had never come up before, since the code that
assigns `root.finishedLanes` is in an extremely hot path, and it hasn't
changed in a while. The reason is that, before 530027a,
`root.finishedLanes` was only used by the DevTools profiler, which is
probably why we had never noticed any issues. In addition to fixing the
inconsistency, we might also consider making `finishedLanes` a
profiling-only field.