This patch replaces IterateBlackObject with two functions:
- RecordWrites,
- ProcessBlackAllocatedObject.

The RecordWrites function is a write barrier, and its behaviour depends
on whether the concurrent marking is on or not.

The ProcessBlackAllocatedObject is the same indepenent from the
concurrent marker.

BUG=chromium:694255

Change-Id: I1666371fbdac9b26c6f875b9e1d1751da4ea1960
Reviewed-on: https://chromium-review.googlesource.com/541441
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46060}