Also moves Mutex to its own file mutex.{cc,h}.

R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/21087012

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@15964 ce2b1a6d-e550-0410-aec6-3dcde31c8c00