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

components/sync_driver/tab_node_pool.cc

Index: components/sync_driver/tab_node_pool.cc
diff --git a/components/sync_driver/tab_node_pool.cc b/components/sync_driver/tab_node_pool.cc
index e3f611961fef497d33087e7374aaf7fc17cf50f4..ee6278ca4a296bf372a1cc1588fbdf0a6393fb8c 100644
--- a/components/sync_driver/tab_node_pool.cc
+++ b/components/sync_driver/tab_node_pool.cc
@@ -28,8 +28,8 @@ 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);
 }