Turn off concurrent sweeping.

R=ulan@chromium.org
BUG=v8:3071
LOG=N

Review URL: https://codereview.chromium.org/116223005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18415 ce2b1a6d-e550-0410-aec6-3dcde31c8c00