cppgc: Unpoison live object before compaction

The object may have been poisoned again between marking and compaction
through executing pre-finalizers or custom weakness handling of
related objects.

Bug: chromium:1220666, chromium:1056170
Change-Id: Ibba4b42852a2921640d6f3ded473521febb2114f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2966386


Reviewed-by: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Auto-Submit: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75189}