Reviewed at http://codereview.chromium.org/160099

The original MacOSMutex constructor makes data race detectors a little
bit crazy. Also, the new version is simpler and easier to understand.



git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@2535 ce2b1a6d-e550-0410-aec6-3dcde31c8c00