Turn on concurrent sweeping.
Turn on concurrent sweeping.

BUG=
R=jarin@chromium.org

Review URL: https://codereview.chromium.org/230733002

git-svn-id: b158db1e4b4ab85d4c9e510fdef4b1e8c614b15b@20612 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh