Enable concurrent sweeping. Added some extra debugging checks for concurrent sweeping.

BUG=
R=titzer@chromium.org

Review URL: https://codereview.chromium.org/138903009

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18722 ce2b1a6d-e550-0410-aec6-3dcde31c8c00