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

Change-Id: I4858d9cbed95e5ca560956b9dabd976cebe68333
diff --git a/runtime/gc/space/zygote_space.cc b/runtime/gc/space/zygote_space.cc
index 0466413..fb3a12e 100644
--- a/runtime/gc/space/zygote_space.cc
+++ b/runtime/gc/space/zygote_space.cc
@@ -115,7 +115,7 @@
     // Need to mark the card since this will update the mod-union table next GC cycle.
     card_table->MarkCard(ptrs[i]);
   }
-  zygote_space->objects_allocated_.FetchAndSub(num_ptrs);
+  zygote_space->objects_allocated_.FetchAndSubSequentiallyConsistent(num_ptrs);
 }
 
 }  // namespace space