This follows up 4af9cfcc by separating incremental marking state
from the full MC marking state. Runtime and tests now use only
the incremental marking state. The full MC marking state used
by MC during atomic pause.

This separation decouples atomicity of markbit accesses
during incremental marking and during full MC.

Bug: chromium:694255
TBR: mlippautz@chromium.org
Change-Id: Ia409ab06515cd0d1403a272a016633295c0d6692
Reviewed-on: https://chromium-review.googlesource.com/612350
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47336}