Now that the main thread has its own LocalHeap, we don't need a
separate instance of MarkingBarrier in Heap for the main thread
anymore. We can just use the MarkingBarrier in
main_thread_local_heap(). This makes code between main and background
threads more uniform.

Bug: v8:13018
Change-Id: I3d2dab1b11815df9a92c2fa7eebf52bf2cb130f2
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3769687Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81841}