Black allocation is already enabled when starting incremental marking.

Bug: v8:12775
Change-Id: I492c3ab89a3a3251ab005d2f3fc4ee46f28f5615
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3820067
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82321}