- Untangles MarkingVisitor from Marker.
- Adds ConservativeTracingVisitor encapsulating conservative
  tracing.

This enables the following architecture:
- Marking visitors (unified + stand-alone) inherit from
  MarkingVisitor;
- Markers (unified + stand-alone) inherit (or directly use) Marker

Bug: chromium:1056170
Change-Id: I05304c231d2983dab5611d05cf4aa8bfa3ed5e20
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2245600Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68366}