Revert of Reland concurrent sweeping of code space. (patchset #6 id:100001 of https...

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=

Review URL: https://codereview.chromium.org/1263343002

Cr-Commit-Position: refs/heads/master@{#29975}