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

sync/test/engine/test_id_factory.h

Index: sync/test/engine/test_id_factory.h
diff --git a/sync/test/engine/test_id_factory.h b/sync/test/engine/test_id_factory.h
index 33347297632af47b65f0bc59dffe93106f7988f7..40a3b5b0f9bf31e2c05123f3ac3e88d7a8c1af76 100644
--- a/sync/test/engine/test_id_factory.h
+++ b/sync/test/engine/test_id_factory.h
@@ -36,12 +36,12 @@ class TestIdFactory {
   }
 
   // Create a local ID from a name.
-  static syncable::Id MakeLocal(std::string name) {
+  static syncable::Id MakeLocal(const std::string& name) {
     return syncable::Id::CreateFromClientString(std::string("lient ") + name);
   }
 
   // Create a server ID from a string name.
-  static syncable::Id MakeServer(std::string name) {
+  static syncable::Id MakeServer(const std::string& name) {
     return syncable::Id::CreateFromServerId(std::string("erver ") + name);
   }