Perform incremental marking step after free-list allocation and clean-up incremental marking start condition.

BUG=

Review URL: https://codereview.chromium.org/14634007

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@14564 ce2b1a6d-e550-0410-aec6-3dcde31c8c00