This reverts commit 7f3f7e8a.

Reason for revert: performance

Original change's description:
> [heap] Use std::atomic for page flags.
> 
> Bug: chromium:852420,chromium:852420
> Change-Id: I873666df415c6b4919f8b3385494c9a08f105188
> Reviewed-on: https://chromium-review.googlesource.com/1170700
> Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
> Commit-Queue: Hannes Payer <hpayer@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#55055}

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

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: chromium:852420
Change-Id: I89fcd085395fa372ae9fb254e55954ff3b7ca4d7
Reviewed-on: https://chromium-review.googlesource.com/1184982Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Hannes Payer <hpayer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55329}