The string forwarding table invokes MakeThin even during a GC.
However during a GC we didn't update the invalidated object size for
OLD_TO_OLD because incremental marking was already stopped at that
point.

Change-Id: Ia2f0d2277b59b299c3c8bf5c3febbde634e3ec06
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3910028Reviewed-by: Patrick Thier <pthier@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#83368}