Reland of land concurrent sweeping of code space. (patchset #1 id:1 of https://codereview.chromium.org/1263343002/)

Reason for revert:
Bogus revert.

Original issue's description:
> Revert of Reland concurrent sweeping of code space. (patchset #6 id:100001 of https://codereview.chromium.org/1242333002/)
>
> Reason for revert:
> Reverted because 507840 came back on recent Chromecrash. Should not have committed this Cl.
>
> Original issue's description:
> > Reland concurrent sweeping of code space.
> >
> > BUG=
> >
> > Committed: https://crrev.com/8516dccf6a561020441773c93c564dd4aa6ee59e
> > Cr-Commit-Position: refs/heads/master@{#29967}
>
> TBR=jochen@chromium.org
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=
>
> Committed: https://crrev.com/5c6e7d00438cc82a5584e3178d7dadf36e4a34f8
> Cr-Commit-Position: refs/heads/master@{#29975}

TBR=jochen@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=

Review URL: https://codereview.chromium.org/1271773002

Cr-Commit-Position: refs/heads/master@{#29976}