Since ManualGCScope changes marking flags it should finalize any
ongoing GC before changing the flags. Otherwise, the GC may observe
inconsistent state.

Bug: chromium:1285706
Change-Id: Ie8ef6a1117ba0523d0bed0c46d9116ffbc02069c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3386607
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Auto-Submit: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78618}