Reland of land concurrent sweeping of code space.

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=

Committed: https://crrev.com/0924d6d542854ef4d78adfd947b1ff2d1cd35778
Cr-Commit-Position: refs/heads/master@{#29976}

[Hannes Payer (out of office), 2015-08-03 14:11:57]

Created Reland of land concurrent sweeping of code space.

[commit-bot: I haz the power, 2015-08-03 14:12:13]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1271773002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1271773002/1

[commit-bot: I haz the power, 2015-08-03 14:12:30]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-08-03 14:12:45]

Patchset 1 (id:??) landed as
https://crrev.com/0924d6d542854ef4d78adfd947b1ff2d1cd35778
Cr-Commit-Position: refs/heads/master@{#29976}