Cleanup Mutex and related classes.

Cleanup Mutex and related classes.

Drop the previous Mutex and ScopedLock classes from platform files.

Add new Mutex, RecursiveMutex and LockGuard classes, which are
designed after their C++11 counterparts, so that at some point
we can simply drop our custom code and switch to the C++11
classes. We distinguish regular and recursive mutexes, as the
latter don't work well with condition variables, which will be
introduced by a followup CL.

R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/23625003

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@16416 ce2b1a6d-e550-0410-aec6-3dcde31c8c00