Revert "Remove sequential sweeping mode and perform lazy sweeping when no sweeper threads are active."

This reverts commit r22346 for breaking GC stress tests.

TBR=hpayer@chromium.org

Review URL: https://codereview.chromium.org/386943003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22349 ce2b1a6d-e550-0410-aec6-3dcde31c8c00