Revert of [heap] Don't unmap new space pages while sweeping is active (patchset #4 id:80001 of https://codereview.chromium.org/2250423002/ )

Reason for revert:
The barrier in newspace is still needed.

Original issue's description:
> [heap] Don't unmap new space pages while sweeping is active
>
> - The barrier for scavenge only checked for whether new space pages were swept.
>   This is not enough as a concurrent task could still hang right before trying to
>   lock the page for sweeping. Remove the barrier completely.
> - Avoid unmapping of new space pages while sweeping using a delayed list that
>   gets emptied upon the next call to the unmapper.
>
> BUG=chromium:628984
> R=hpayer@chromium.org
>
> Committed: https://crrev.com/982b399423e6bd941cabb2b825031cd8d5eb4980
> Cr-Commit-Position: refs/heads/master@{#38710}

TBR=hpayer@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=chromium:628984

Review-Url: https://codereview.chromium.org/2244233007
Cr-Commit-Position: refs/heads/master@{#38712}