[heap] Disable concurrent marking.

It was accidentally enabled in 82202251

Bug: chromium:694255
TBR: mlippautz@chromium.org
Change-Id: I7febb8528a5116cfd43efdc41208db33841da495
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/603308Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47188}