dillon1000/react

Quick fix for minor typo in ReactScheduler (#12834)

**what is the change?:**
We were setting a flag after some early returns, should have set it
right away.

To be fair, it's not clear how you can hit a problem with the current
state of things. Even if a callback is cancelled, it's still in the
'pendingCallbacks' queue until the rAF runs, and we only schedule a rAF
when there are pendingCallbacks in the queue.

But since this is obviously wrong, going to fix it.

We will be adding a regression test in a follow-up PR.

**why make this change?:**
To fix a random bug which was popping up.

**test plan:**
Adding a regression unit test in a follow-up PR.