This reverts commit a9a50dc9.

Reason for revert: buildbot crashes.

Original change's description:
> [heap] Temporarily disable concurrent marking.
> 
> The 6.3 branch has a data race that is fixed in 6.4 but the fix is too
> large for back merging.
> 
> This CL will be back-merged to 6.3 after getting Canary coverage.
> 
> Concurrent marking will be re-enabled afterwards.
> 
> Bug: chromium:774644
> Change-Id: I4112da0e133a637cc4fb52dee2e4c165cdc74f1f
> Reviewed-on: https://chromium-review.googlesource.com/749811
> Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
> Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#49080}

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

Change-Id: Ia9d2128c01b811073c1c8f0392eb13b7d7745cd1
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: chromium:774644
Reviewed-on: https://chromium-review.googlesource.com/751501Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49083}