bug 1321884 - clean up the HeldMutexStack on helper threads r=jonco

The HeldMutexStack is cleaned up in JS_ShutDown() on the main thread, but since
it is thread local we need to clean it up on other threads where it is used.

js/src/vm/HelperThreads.cpp

@@ -1407,6 +1407,7 @@ HelperThread::ThreadMain(void* arg)
     FIX_FPU();
 
     static_cast<HelperThread*>(arg)->threadLoop();
+    Mutex::ShutDown();
 }
 
 void