Split off MarkingState from MarkingVisitor.

With this CL the marking implementation is moved to "MarkingState"
which is the new bottleneck for marking a single object.
MarkingVisitor merely forwards to MarkingState, which knows how to set
the markbit and add the object to the worklist accordingly. This
allows to have a "UnifiedHeapMarkingVisitor" in future which can
easily reuse Marking to provide C++ marking.

Change-Id: I87ebbe37e8e8cd841e872cae9dc3490e2b55c4dd
Bug: chromium:1056170
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2270172Reviewed-by: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68660}