This reverts commit c41c7a09.

Reason for revert: relanding now that the perf waterfall has had a stab at this revert.

Original change's description:
> Revert "Use all available workers for concurrent marking."
> 
> This reverts commit 3c62f7ae.
> (and commit 4939463c)
> 
> The goal of this revert is to contrast the effect on perf bots of
> landing it vs reverting it to more easily attribute its impact.
> 
> R=​hpayer@chromium.org
> 
> Bug: chromium:812178
> Change-Id: I7c977b1b0b587f787263272400d87f6aae7af634
> Reviewed-on: https://chromium-review.googlesource.com/936761
> Commit-Queue: Hannes Payer <hpayer@chromium.org>
> Reviewed-by: Hannes Payer <hpayer@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#51546}

TBR=gab@chromium.org,hpayer@chromium.org

Change-Id: I1ecfc70867dc5424cba1a9ecd229ae031c3e9aa4
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: chromium:812178
Reviewed-on: https://chromium-review.googlesource.com/937725Reviewed-by: Hannes Payer <hpayer@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51577}