CompleteInobjectSlackTracking potentially shrinks multiple maps, and
the relation between these maps should be preserved in a concurrent
environment. Thus it is not enough to make each modification
atomically, but all related map modifications must be within a
critical section.

We do this by locking the map_updater_access mutex
CompleteInobjectSlackTracking, and hence moving the function to the
MapUpdater class.

Bug: chromium:1274445,v8:7990
Change-Id: If99bb8b55e03180128ee397d845fa4c269c4241e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3379819Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78597}