[Sync] Pass ModelTypeSet by value everywhere

sync/notifier/invalidation_util.cc

Index: sync/notifier/invalidation_util.cc
diff --git a/sync/notifier/invalidation_util.cc b/sync/notifier/invalidation_util.cc
index 1c6b931f737f2b6b0321a6dc3a48e4e390512677..7cc80d299ba2ab9e21e9777eabcb80f8a9c57772 100644
--- a/sync/notifier/invalidation_util.cc
+++ b/sync/notifier/invalidation_util.cc
@@ -72,7 +72,7 @@ std::string ObjectIdToString(
   return str;
 }
 
-ObjectIdSet ModelTypeSetToObjectIdSet(const ModelTypeSet& model_types) {
+ObjectIdSet ModelTypeSetToObjectIdSet(ModelTypeSet model_types) {
   ObjectIdSet ids;
   for (ModelTypeSet::Iterator it = model_types.First(); it.Good(); it.Inc()) {
     invalidation::ObjectId model_type_as_id;