Access to the object start bitmap is only safe during marking until
sweeping is started as the concurrent sweeper may clear and rebuild
the bitmap at any time during sweeping.

Adds a DCHECK and an additional test for a previously broken
pre-finalizer scenario.

Bug: chromium:1307471
Change-Id: If67ade43f7cdad6de4720c0efeac11bfe8c22b3c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3535782Reviewed-by: Nikolaos Papaspyrou <nikolaos@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79550}