This moves marking write barrier related functions from Heap and
IncrementalMarking into a separate class: MarkingBarrier.

Additionally, a new WriteBarrier class is added at the heap API level
that dispatches to MarkingBarrier.

Future CLs will move slots recording in MarkingBarrier and apply
the same refactoring to the generational barrier. An instance of
MarkingBarrier will be added to each LocalHeap and enable it to
emit a write barrier from a background thread.

Bug: v8:10315
Change-Id: Icc147b48563d88c85d99ead99b1e201f523721d0
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2280083Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68703}