[Sync] Refactor sync configuration logic.

chrome/browser/sync/glue/backend_data_type_configurer.h

Index: chrome/browser/sync/glue/backend_data_type_configurer.h
diff --git a/chrome/browser/sync/glue/backend_data_type_configurer.h b/chrome/browser/sync/glue/backend_data_type_configurer.h
index 161bc1437b9d6c0d0bc198dd545581c9a9fb8834..f15986bd54cdd1a470c2013147a7889598183c1f 100644
--- a/chrome/browser/sync/glue/backend_data_type_configurer.h
+++ b/chrome/browser/sync/glue/backend_data_type_configurer.h
@@ -34,11 +34,11 @@ class BackendDataTypeConfigurer {
   // Nigori.
   virtual void ConfigureDataTypes(
       sync_api::ConfigureReason reason,
-      syncable::ModelTypeSet types_to_add,
-      syncable::ModelTypeSet types_to_remove,
+      const syncable::ModelTypeSet& types_to_add,
+      const syncable::ModelTypeSet& types_to_remove,
       NigoriState nigori_state,
-      base::Callback<void(syncable::ModelTypeSet)> ready_task,
-      base::Callback<void()> retry_callback) = 0;
+      const base::Callback<void(syncable::ModelTypeSet)>& ready_task,
+      const base::Callback<void()>& retry_callback) = 0;
 
  protected:
   virtual ~BackendDataTypeConfigurer() {}