Set unswept free bytes for concurent sweeper.

BUG=

Review URL: https://codereview.chromium.org/12184016

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13776 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Turn off parallel and concurrent sweeping on system with just one processor/core.

BUG=

Review URL: https://codereview.chromium.org/12380020

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13777 ce2b1a6d-e550-0410-aec6-3dcde31c8c00