This is the first step in unification of concurrent and main thread
marking visitors. The new MarkingVisitorBase will become a base class
for all marking visitors and will remove the existing code duplication.

This is a refactoring without behavior change.

Subsequent CL will change the main thread marking visitor to derive
from the new base class.

Bug: chromium:1019218

Change-Id: I3d47030d396e0ba6706882fbd922bbcac46181b2
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1886920Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64632}