Revert of Reland concurrent sweeping of code space.

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}

[Hannes Payer (out of office), 2015-08-03 13:06:30]

Created Revert of Reland concurrent sweeping of code space.

[commit-bot: I haz the power, 2015-08-03 13:06:38]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1263343002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1263343002/1

[commit-bot: I haz the power, 2015-08-03 13:06:48]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-08-03 13:07:05]

Patchset 1 (id:??) landed as
https://crrev.com/5c6e7d00438cc82a5584e3178d7dadf36e4a34f8
Cr-Commit-Position: refs/heads/master@{#29975}

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

A revert of this CL (patchset #1 id:1) has been created in
https://codereview.chromium.org/1271773002/ by hpayer@chromium.org.

The reason for reverting is: Bogus revert..