This reverts commit 33d5006e.

Reason for revert: Still failing TSAN: https://build.chromium.org/p/client.v8/builders/V8%20Linux64%20TSAN/builds/15527/steps/Check%20-%20extra%20%28flakes%29/logs/regress-470804

Original change's description:
> Reland "[heap] Move pointers updating to ItemParallelJob""
> 
> Furthermore avoid lock-step between pointer updating phases as they
> should execute in parallel without synchronization restrictions.
> 
> This reverts commit 68a72399.
> 
> Bug: chromium:726040
> Change-Id: I76bb31d146f8aa20f8b7c486eeae9f09efa0cd53
> Reviewed-on: https://chromium-review.googlesource.com/518150
> Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
> Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#45644}

TBR=ulan@chromium.org,mlippautz@chromium.org

No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: chromium:726040
Change-Id: I96090250a6ecc5068c3c927e8435e0b605c3c335
Reviewed-on: https://chromium-review.googlesource.com/519409
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45645}