[heap] Only start incremental marking when V8 is not in GC VM state.

Bug: v8:12503
Change-Id: Icda291d9770c46c7fee3c70dd4df97f320b1956a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3398113


Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Hannes Payer <hpayer@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79623}