[Sync] Pass ModelTypeSet by value everywhere

sync/notifier/invalidation_util.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 //
 // Various utilities for dealing with invalidation data types.
 
 #ifndef SYNC_NOTIFIER_INVALIDATION_UTIL_H_
 #define SYNC_NOTIFIER_INVALIDATION_UTIL_H_
 
 #include <iosfwd>
@@ skipping 34 matching lines @@
 
 // Caller owns the returned DictionaryValue.
 scoped_ptr<base::DictionaryValue> ObjectIdToValue(
     const invalidation::ObjectId& object_id);
 
 bool ObjectIdFromValue(const base::DictionaryValue& value,
                        invalidation::ObjectId* out);
 
 std::string ObjectIdToString(const invalidation::ObjectId& object_id);
 
-ObjectIdSet ModelTypeSetToObjectIdSet(const ModelTypeSet& models);
+ObjectIdSet ModelTypeSetToObjectIdSet(ModelTypeSet models);
 ModelTypeSet ObjectIdSetToModelTypeSet(const ObjectIdSet& ids);
 
 std::string InvalidationToString(
     const invalidation::Invalidation& invalidation);
 
 }  // namespace syncer
 
 #endif  // SYNC_NOTIFIER_INVALIDATION_UTIL_H_