Make thread_creation_mutex a Thread::PlatformData member.

BUG=
R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/198643003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19933 ce2b1a6d-e550-0410-aec6-3dcde31c8c00