This is a reland of 8c4a8250
Original change's description:
> [heap] Enable concurrent marking for x86 and x64.
> 
> Bug: chromium:694255
> Change-Id: I28c8c6e5ba6c84123f3951e822c132860cb22c1d
> Reviewed-on: https://chromium-review.googlesource.com/641451
> Commit-Queue: Kentaro Hara <haraken@chromium.org>
> Reviewed-by: Michael Achenbach <machenbach@chromium.org>
> Reviewed-by: Hannes Payer (slow) <hpayer@chromium.org>
> Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#48100}

Bug: chromium:694255
TBR: mlippautz@chromium.org
Change-Id: Ic36515dbd418c219bccbbf371126a4dfd66a466f
Reviewed-on: https://chromium-review.googlesource.com/676966Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48107}