Parallel and concurrent sweeping.

Sweep old pointer space and old data space concurrently to the main mutator thread and in parallel.

BUG=

Review URL: https://codereview.chromium.org/11782028

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13552 ce2b1a6d-e550-0410-aec6-3dcde31c8c00