[Sync] Start implementation of migration for USS.

components/sync/engine_impl/uss_migrator.h

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef COMPONENTS_SYNC_ENGINE_IMPL_USS_MIGRATOR_H_
+#define COMPONENTS_SYNC_ENGINE_IMPL_USS_MIGRATOR_H_
+
+#include "components/sync/base/model_type.h"
+
+namespace syncer {
+
+class ModelTypeWorker;
+struct UserShare;
+
+// Pulls all the data for |type| out of the directory and sends it to |worker|
+// as the result of an initial GetUpdates. |batch_size| is only a parameter so
+// it can be lowered for unit testing. Returns whether migration was a success.
+bool MigrateDirectoryData(ModelType type,
+                          UserShare* user_share,
+                          ModelTypeWorker* worker,
+                          int batch_size = 64);
+
+}  // namespace syncer
+
+#endif  // COMPONENTS_SYNC_ENGINE_IMPL_USS_MIGRATOR_H_