- Make sure we use fast prechecks in the header files
- MicrotaskQueue::CallEnqueueMicrotask returns a Smi instead of a
  more costly undefined value (the return value is enforced by the
  calling convention, but unused)
- Merge FireMicrotasksCompletedCallback into OnComplete

Change-Id: I3797b946bcffb6349e5693c41478bd2bad1f93fb
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3024154
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Victor Gomes <victorgomes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75985}