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

Change-Id: I4858d9cbed95e5ca560956b9dabd976cebe68333
diff --git a/runtime/gc/collector/mark_sweep.h b/runtime/gc/collector/mark_sweep.h
index e9a3c3a..d73bf3f 100644
--- a/runtime/gc/collector/mark_sweep.h
+++ b/runtime/gc/collector/mark_sweep.h
@@ -305,14 +305,14 @@
   AtomicInteger mark_fastpath_count_;
   AtomicInteger mark_slowpath_count_;
 
-  // Verification.
-  size_t live_stack_freeze_size_;
-
   std::unique_ptr<Barrier> gc_barrier_;
   Mutex mark_stack_lock_ ACQUIRED_AFTER(Locks::classlinker_classes_lock_);
 
   const bool is_concurrent_;
 
+  // Verification.
+  size_t live_stack_freeze_size_;
+
  private:
   friend class AddIfReachesAllocSpaceVisitor;  // Used by mod-union table.
   friend class CardScanTask;