Directly set the value in the descriptors pointer to avoid checks.

This is the only place from where we are allowed to modify the value of
the descriptors pointer.

Review URL: https://chromiumcodereview.appspot.com/10905266

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@12500 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/objects.cc

@@ -1810,7 +1810,8 @@ MaybeObject* JSObject::ConvertTransitionToMapTransition(
     // new_map and install its descriptors in the old_map. Since the old_map
     // stores the descriptors for the new_map, remove the transition array of
     // the new_map that is only in place to store the descriptors.
-    old_map->transitions()->set_descriptors(new_map->instance_descriptors());
+    old_map->transitions()->descriptors_pointer()->set_value(
+        new_map->instance_descriptors());
     new_map->ClearTransitions(GetHeap());
     old_map->set_owns_descriptors(false);
     Map* map;