"Finishing" currently only means decrementing a counter and notifying
users via the callback if baseline compilation or tier-up finished.
We can totally do this from background, if we spawn foreground tasks to
actually call the callback.

R=ahaas@chromium.org

Bug: v8:7921, v8:8423
Change-Id: I815964d07a4d9111248097c41ebbd650842304c3
Cq-Include-Trybots: luci.v8.try:v8_linux64_tsan_rel
Reviewed-on: https://chromium-review.googlesource.com/c/1375662Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58547}