[Sync] Convert DTCs to be not RefCounted and NonThreadSafe.

components/sync/driver/model_association_manager.cc

Index: components/sync/driver/model_association_manager.cc
diff --git a/components/sync/driver/model_association_manager.cc b/components/sync/driver/model_association_manager.cc
index 55d59286000fdff6525982f488b576fe276d8d79..a071464a38c3b848eb28f33c58ad43b94787355c 100644
--- a/components/sync/driver/model_association_manager.cc
+++ b/components/sync/driver/model_association_manager.cc
@@ -262,8 +262,9 @@ void ModelAssociationManager::Stop() {
   }
 }
 
-void ModelAssociationManager::ModelLoadCallback(syncer::ModelType type,
-                                                syncer::SyncError error) {
+void ModelAssociationManager::ModelLoadCallback(
+    syncer::ModelType type,
+    const syncer::SyncError& error) {
   DVLOG(1) << "ModelAssociationManager: ModelLoadCallback for "
            << syncer::ModelTypeToString(type);