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

components/sync_driver/tab_node_pool.cc

 // 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.
 
 #include "components/sync_driver/tab_node_pool.h"
 
 #include "base/format_macros.h"
 #include "base/logging.h"
 #include "base/strings/stringprintf.h"
 #include "sync/api/sync_change.h"
@@ skipping 10 matching lines @@
 TabNodePool::TabNodePool()
     : max_used_tab_node_id_(kInvalidTabNodeID) {}
 
 // static
 // We start vending tab node IDs at 0.
 const int TabNodePool::kInvalidTabNodeID = -1;
 
 TabNodePool::~TabNodePool() {}
 
 // Static
-std::string TabNodePool::TabIdToTag(
-    const std::string machine_tag, int tab_node_id) {
+std::string TabNodePool::TabIdToTag(const std::string& machine_tag,
+                                    int tab_node_id) {
   return base::StringPrintf("%s %d", machine_tag.c_str(), tab_node_id);
 }
 
 void TabNodePool::AddTabNode(int tab_node_id) {
   DCHECK_GT(tab_node_id, kInvalidTabNodeID);
   DCHECK(nodeid_tabid_map_.find(tab_node_id) == nodeid_tabid_map_.end());
   unassociated_nodes_.insert(tab_node_id);
   if (max_used_tab_node_id_ < tab_node_id)
     max_used_tab_node_id_ = tab_node_id;
 }
@@ skipping 136 matching lines @@
 
 bool TabNodePool::Empty() const { return free_nodes_pool_.empty(); }
 
 bool TabNodePool::Full() { return nodeid_tabid_map_.empty(); }
 
 void TabNodePool::SetMachineTag(const std::string& machine_tag) {
   machine_tag_ = machine_tag;
 }
 
 }  // namespace browser_sync