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

components/sync/driver/model_association_manager.h

Index: components/sync/driver/model_association_manager.h
diff --git a/components/sync/driver/model_association_manager.h b/components/sync/driver/model_association_manager.h
index 89095d408bbad541903780aa43a8e00d873020a6..0c4092a5035a6cffbcde4acb10fb6684e1299365 100644
--- a/components/sync/driver/model_association_manager.h
+++ b/components/sync/driver/model_association_manager.h
@@ -119,7 +119,8 @@ class ModelAssociationManager {
 
   // Callback that will be invoked when the models finish loading. This callback
   // will be passed to |LoadModels| function.
-  void ModelLoadCallback(syncer::ModelType type, syncer::SyncError error);
+  void ModelLoadCallback(syncer::ModelType type,
+                         const syncer::SyncError& error);
 
   // Called when all requested types are associated or association times out.
   // Will clean up any unfinished types, and update |state_| to be |new_state|