Reland concurrent sweeping of code space.

Reland concurrent sweeping of code space.

BUG=chromium:506778, chromium:506957, chromium:507211
LOG=n

Committed: https://crrev.com/806b81f11e3bfaef0d4330c7669e6934074be9cb
Cr-Commit-Position: refs/heads/master@{#29748}

[Hannes Payer (out of office), 2015-07-06 09:50:29]

hpayer@chromium.org changed reviewers:
+ jochen@chromium.org, mvstanton@chromium.org

[mvstanton, 2015-07-06 09:51:53]

lgtm

[Hannes Payer (out of office), 2015-07-20 10:34:45]

The CQ bit was checked by hpayer@chromium.org

[Hannes Payer (out of office), 2015-07-20 10:34:46]

The patchset sent to the CQ was uploaded after l-g-t-m from
mvstanton@chromium.org
Link to the patchset: https://codereview.chromium.org/1225733002/#ps80001
(title: " ")

[commit-bot: I haz the power, 2015-07-20 10:34:55]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1225733002/80001

[commit-bot: I haz the power, 2015-07-20 10:36:41]

Committed patchset #5 (id:80001)

[commit-bot: I haz the power, 2015-07-20 10:36:58]

Patchset 5 (id:??) landed as
https://crrev.com/806b81f11e3bfaef0d4330c7669e6934074be9cb
Cr-Commit-Position: refs/heads/master@{#29748}

[Hannes Payer (out of office), 2015-07-20 12:06:49]

A revert of this CL (patchset #5 id:80001) has been created in
https://codereview.chromium.org/1242743002/ by hpayer@chromium.org.

The reason for reverting is: Tests became flaky..