This removes object visiting logic from IncrementalMarking and makes it
call the corresponding methods of MarkCompactCollector. As a result
we have one place where objects are visited (on the main thread), which
is necessary for implementing per-context visitation.

Bug: chromium:973627
Change-Id: Ibdfbb9a910b592307bdba2bd73eada35c80a0d61
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1940154Reviewed-by: Hannes Payer <hpayer@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65278}