So far, we always finished the baseline units before finishing any
tiering unit. This will be refactored to finish all units from the
background threads, so the finishing can happen in any order.
Thus refactor the counters to count both separately, and trigger the
right events.

R=ahaas@chromium.org

Bug: v8:7921
Change-Id: Ia2d8ab3f70f9bc3406eff428da5d22580558887b
Reviewed-on: https://chromium-review.googlesource.com/c/1333669
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57608}