This reverts commit 81d3427b.

Reason for revert: Several gc related failures, e.g.:
https://build.chromium.org/p/client.v8/builders/V8%20Mac%20GC%20Stress/builds/13488

Original change's description:
> [heap] Move pointers updating to ItemParallelJob
> 
> Furthermore avoid lock-step between pointer updating phases as they
> should execute in parallel without synchronization restrictions.
> 
> Bug: chromium:726040
> Change-Id: I26ce0d1f2a4637ff5610cae556113e3d736788e2
> Reviewed-on: https://chromium-review.googlesource.com/518103
> Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
> Reviewed-by: Hannes Payer <hpayer@chromium.org>
> Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#45574}

NOPRESUBMIT=true
NOTRY=true
NOTREECHECKS=true

TBR=ulan@chromium.org,hpayer@chromium.org,mlippautz@chromium.org
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: chromium:726040

Change-Id: I60860aef390605d07bc520141cab9d5be9b712b3
Reviewed-on: https://chromium-review.googlesource.com/518106
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45575}