On concurrent threads, CppMarkingState allocates its own
cppgc::internal::MarkingStateBase.
On the mutator thread, CppMarkingState reuses the same MarkingStateBase
as CppHeap's mutator thread visitor.
That means the mutator thread doesn't need to rely on publishing
segments to push object from V8 to CppHeap.

Bug: v8:12407
Change-Id: I161adf8dcdc9aa960de65b47feb2abd3b605df7c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3295454Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78449}