Now incremental marker and stop-the-world marker use the same visitor,
which is derived from MarkingVisitorBase. This removes code duplication
and also should reduce binary size.

The marking worklist processing code also changes to not color the
object black before visiting it. Instead the visitor colors the
object black in ShouldVisit method.

Bug: chromium:1019218
Change-Id: I57971122f3c77ad2770b6754d696b79d802ef1a4
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1901271
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64848}