This CL introduces:
- Worklist
- MarkingHandler to manage gc marking phase
- Integration into CollectGarbage for atomic pause GC
- MarkingVisitor for main thread marking

Still missing from this CL:
- Proper handling for stack scanning
- Handling of previously not fully constructed objects

Bug: chromium:1056170
Change-Id: I70ac8534dfb898777cf3a06e3119cac8072174fd
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2170526
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67642}