This CL introduces concurrent marking to the cppgc library.
The CL includes:
(*) Split MarkingState to mutator thread and concurrent thread.
(*) Split MarkingVisitor to mutator thread and concurrent thread.
(*) Introduce ConcurrentMarker for managing concurrent marking.
(*) Update unified heap to support concurrent marking as well.

See slides 13 and 14 in the following link for class hierarchies:
https://docs.google.com/presentation/d/1uDiEjJ-f1VziBKmYcvpw2gglP47M53bwj1L-P__l9QY/

Bug: chromium:1056170
Change-Id: I6530c2b21613011a612773d36fbf37416c23c5e7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2424348
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70352}