Reviewed by Anders Carlsson.

        https://bugs.webkit.org/show_bug.cgi?id=26373

        Add a new class to Threading in wtf called ReadWriteLock that handles single writer/multiple reader locking.
        Provide a pthreads-only implementation of the lock for now, as this class is only going to be used
        on Snow Leopard at first.

        * wtf/Threading.h:
        (WTF::ReadWriteLock::impl):
        * wtf/ThreadingPthreads.cpp:
        (WTF::ReadWriteLock::ReadWriteLock):
        (WTF::ReadWriteLock::~ReadWriteLock):
        (WTF::ReadWriteLock::readLock):
        (WTF::ReadWriteLock::tryReadLock):
        (WTF::ReadWriteLock::writeLock):
        (WTF::ReadWriteLock::tryWriteLock):
        (WTF::ReadWriteLock::unlock):



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@44651 268f45cc-cd09-0410-ab3c-d52691b4dbfc