Split off ConservativeMarkingVisitor from MarkingVisitor.

After this change, MarkingVisitor and ConservativeMarkingVisitor are
types that are instantiated with Marking and merely forward to
MarkingState accrodingly. The two marking-related visitors can be
passed along as interface types cppgc::Visitor and
ConservativeTracingVisitor, respectively.

Change-Id: Iad103dc3053c61d1a104a8802edd420d21cdf935
Bug: chromium:1056170
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2270538
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68661}