Synchronize store buffer processing and concurrent sweeping.

BUG=
R=jarin@chromium.org, mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/227533006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20582 ce2b1a6d-e550-0410-aec6-3dcde31c8c00