Begin migration of art::Atomic to std::atomic.

Change-Id: I4858d9cbed95e5ca560956b9dabd976cebe68333
diff --git a/runtime/base/mutex-inl.h b/runtime/base/mutex-inl.h
index a7e25cb..adf4c66 100644
--- a/runtime/base/mutex-inl.h
+++ b/runtime/base/mutex-inl.h
@@ -221,7 +221,7 @@
       // Reduce state by 1.
       done = android_atomic_release_cas(cur_state, cur_state - 1, &state_) == 0;
       if (done && (cur_state - 1) == 0) {  // cas may fail due to noise?
-        if (num_pending_writers_ > 0 || num_pending_readers_ > 0) {
+        if (num_pending_writers_.LoadRelaxed() > 0 || num_pending_readers_ > 0) {
           // Wake any exclusive waiters as there are now no readers.
           futex(&state_, FUTEX_WAKE, -1, NULL, NULL, 0);
         }