This reverts commit 8bbc2242.

Reason for revert: On Canary 3195.

Original change's description:
> [heap] Enable concurrent marking for x86 and x64.
> 
> This is an experiment and will be reverted after getting canary
> coverage.
> 
> Bug: chromium:694255
> Change-Id: I40388d8c6db0e46e2ce64e88aba04c5ac8822e94
> Reviewed-on: https://chromium-review.googlesource.com/625959
> Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
> Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#47541}

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

Change-Id: I642c1f778267a795bf1e1a6bba863552394ad1d4
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: chromium:694255
Reviewed-on: https://chromium-review.googlesource.com/631717Reviewed-by: Michael Hablich <hablich@chromium.org>
Commit-Queue: Michael Hablich <hablich@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47564}