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

components/sync_driver/tab_node_pool.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 COMPONENTS_SYNC_DRIVER_TAB_NODE_POOL_H_
 #define COMPONENTS_SYNC_DRIVER_TAB_NODE_POOL_H_
 
 #include <map>
 #include <set>
 #include <string>
@@ skipping 38 matching lines @@
   // If free nodes > kFreeNodesHighWatermark, delete all free nodes until
   // free nodes <= kFreeNodesLowWatermark.
   static const size_t kFreeNodesLowWatermark;
 
   // Maximum limit of FreeNodes allowed on the client.
   static const size_t kFreeNodesHighWatermark;
 
   static const int kInvalidTabNodeID;
 
   // Build a sync tag from tab_node_id.
-  static std::string TabIdToTag(const std::string machine_tag,
+  static std::string TabIdToTag(const std::string& machine_tag,
                                 int tab_node_id);
 
   // Returns the tab_node_id for the next free tab node. If none are available,
   // creates a new tab node and adds it to free nodes pool. The free node can
   // then be used to associate with a tab by calling AssociateTabNode.
   // Note: The node is considered free until it has been associated. Repeated
   // calls to GetFreeTabNode will return the same id until node has been
   // associated.
   // |change_output| *must* be provided. It is the TabNodePool's link to
   // the SyncChange pipeline that exists in the caller context. If the need
@@ skipping 86 matching lines @@
   // The machine tag associated with this tab pool. Used in the title of new
   // sync nodes.
   std::string machine_tag_;
 
   DISALLOW_COPY_AND_ASSIGN(TabNodePool);
 };
 
 }  // namespace browser_sync
 
 #endif  // COMPONENTS_SYNC_DRIVER_TAB_NODE_POOL_H_