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

Change-Id: I4858d9cbed95e5ca560956b9dabd976cebe68333
diff --git a/runtime/gc/space/bump_pointer_space-inl.h b/runtime/gc/space/bump_pointer_space-inl.h
index 497a61f..71c295e 100644
--- a/runtime/gc/space/bump_pointer_space-inl.h
+++ b/runtime/gc/space/bump_pointer_space-inl.h
@@ -48,8 +48,8 @@
   end_ += num_bytes;
   *bytes_allocated = num_bytes;
   // Use the CAS free versions as an optimization.
-  objects_allocated_ = objects_allocated_ + 1;
-  bytes_allocated_ = bytes_allocated_ + num_bytes;
+  objects_allocated_.StoreRelaxed(objects_allocated_.LoadRelaxed() + 1);
+  bytes_allocated_.StoreRelaxed(bytes_allocated_.LoadRelaxed() + num_bytes);
   if (UNLIKELY(usable_size != nullptr)) {
     *usable_size = num_bytes;
   }
@@ -76,8 +76,8 @@
 inline mirror::Object* BumpPointerSpace::AllocNonvirtual(size_t num_bytes) {
   mirror::Object* ret = AllocNonvirtualWithoutAccounting(num_bytes);
   if (ret != nullptr) {
-    objects_allocated_.FetchAndAdd(1);
-    bytes_allocated_.FetchAndAdd(num_bytes);
+    objects_allocated_.FetchAndAddSequentiallyConsistent(1);
+    bytes_allocated_.FetchAndAddSequentiallyConsistent(num_bytes);
   }
   return ret;
 }