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

Reason: broke win64 build

This reverts commit 221bfdd2da2b6f3c1cbe77c5d197f1ea626b0bd2.

TBR=hpayer@chromium.org
BUG=

Review URL: https://codereview.chromium.org/393523002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22374 ce2b1a6d-e550-0410-aec6-3dcde31c8c00