The fence ensures that the concurrent marker observes consistent state
of mark-bits for newly allocated objects.

The patch also moves Bitmap functions to cc file and removes non-atomic
versions of SetRange and ClearRange.

BUG=chromium:694255

Change-Id: I466bef654f3d4a21b7aaebdfd6d5a39ddb5f2a0a
Reviewed-on: https://chromium-review.googlesource.com/530367Reviewed-by: Hannes Payer <hpayer@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45868}