Cleanup: Pass std::string as const reference if possible

sync/test/fake_server/bookmark_entity.h

 // Copyright 2014 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.
 
 #ifndef SYNC_TEST_FAKE_SERVER_BOOKMARK_ENTITY_H_
 #define SYNC_TEST_FAKE_SERVER_BOOKMARK_ENTITY_H_
 
 #include <map>
 #include <string>
 
@@ skipping 31 matching lines @@
                  const std::string& name,
                  const std::string& originator_cache_guid,
                  const std::string& originator_client_item_id,
                  const sync_pb::UniquePosition& unique_position,
                  const sync_pb::EntitySpecifics& specifics,
                  bool is_folder,
                  const std::string& parent_id,
                  int64 creation_time,
                  int64 last_modified_time);
 
-  void SetParentId(std::string parent_id);
+  void SetParentId(const std::string& parent_id);
 
   // FakeServerEntity implementation.
   std::string GetParentId() const override;
   void SerializeAsProto(sync_pb::SyncEntity* proto) const override;
   bool IsFolder() const override;
 
  private:
   // All member values have equivalent fields in SyncEntity.
   std::string originator_cache_guid_;
   std::string originator_client_item_id_;
   sync_pb::UniquePosition unique_position_;
   bool is_folder_;
   std::string parent_id_;
   int64 creation_time_;
   int64 last_modified_time_;
 };
 
 }  // namespace fake_server
 
 #endif  // SYNC_TEST_FAKE_SERVER_BOOKMARK_ENTITY_H_