This reverts commit feb3ee40.

Reason for revert: canary crashes were caused by another CL.

Original change's description:
> [heap] Disable parallel scavenge.
> 
> Bug: chromium:752750
> TBR: mlippautz@chromium.org
> Change-Id: I95ca30742c05be03dcaebb0cd32af04b226e23e2
> Reviewed-on: https://chromium-review.googlesource.com/603652
> Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
> Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#47201}

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

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: chromium:752750
Change-Id: Id26e58ded32af364b3274d6d1a7215944386bb70
Reviewed-on: https://chromium-review.googlesource.com/607891Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47244}