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

Change-Id: I4858d9cbed95e5ca560956b9dabd976cebe68333
diff --git a/runtime/monitor.h b/runtime/monitor.h
index bc5d2e4..bc1b2ed4 100644
--- a/runtime/monitor.h
+++ b/runtime/monitor.h
@@ -107,7 +107,7 @@
   bool IsLocked() SHARED_LOCKS_REQUIRED(Locks::mutator_lock_);
 
   bool HasHashCode() const {
-    return hash_code_.Load() != 0;
+    return hash_code_.LoadRelaxed() != 0;
   }
 
   MonitorId GetMonitorId() const {