[Sync] Start implementation of migration for USS.

components/sync/engine_impl/model_type_worker.h

Index: components/sync/engine_impl/model_type_worker.h
diff --git a/components/sync/engine_impl/model_type_worker.h b/components/sync/engine_impl/model_type_worker.h
index 70f4925cef2f3d40e1c710199a844fe659c02c07..2d9b93b59280b13590178699f64c8c8e287a8988 100644
--- a/components/sync/engine_impl/model_type_worker.h
+++ b/components/sync/engine_impl/model_type_worker.h
@@ -60,6 +60,7 @@ class ModelTypeWorker : public UpdateHandler,
  public:
   ModelTypeWorker(ModelType type,
                   const sync_pb::ModelTypeState& initial_state,
+                  bool trigger_initial_sync,
                   std::unique_ptr<Cryptographer> cryptographer,
                   NudgeHandler* nudge_handler,
                   std::unique_ptr<ModelTypeProcessor> model_type_processor,
@@ -97,6 +98,10 @@ class ModelTypeWorker : public UpdateHandler,
   // Callback for when our contribution gets a response.
   void OnCommitResponse(CommitResponseDataList* response_list);
 
+  // If migration the directory encounters an error partway through, we need to
+  // clear the update data that has been added so far.
+  void AbortMigration();
+
   base::WeakPtr<ModelTypeWorker> AsWeakPtr();
 
  private: