During minor MC new space evacuation we could have two adjacent LABs
sharing a mark bit cell. The DCHECK when observing and changing markbits
of the target needs to reflect that.

Bug: chromium:651354
Change-Id: I737d0f9e3d37dfb1cda3f126d37ed5e7123bedc9
Reviewed-on: https://chromium-review.googlesource.com/541296Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46032}