[Sync] Start implementation of migration for USS.

components/sync/engine_impl/uss_migrator.h

Index: components/sync/engine_impl/uss_migrator.h
diff --git a/components/sync/engine_impl/uss_migrator.h b/components/sync/engine_impl/uss_migrator.h
new file mode 100644
index 0000000000000000000000000000000000000000..28cdddb44535f3c8b693fa8f09973b0620472f0f
--- /dev/null
+++ b/components/sync/engine_impl/uss_migrator.h
@@ -0,0 +1,34 @@
+// 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 "base/callback.h"
+#include "components/sync/base/model_type.h"
+
+namespace syncer {
+
+class ModelTypeWorker;
+struct UserShare;
+
+typedef base::Callback<bool(ModelType, UserShare*, ModelTypeWorker*)>
+    UssMigrator;
+
+// Pulls all the data for |type| out of the directory and sends it to |worker|
+// as the result of an initial GetUpdates. Returns whether migration succeeded.
+bool MigrateDirectoryData(ModelType type,
+                          UserShare* user_share,
+                          ModelTypeWorker* worker);
+
+// A version of the above with |batch_size| as a parameter so it can be lowered
+// for unit testing.
+bool MigrateDirectoryDataWithBatchSize(ModelType type,
+                                       UserShare* user_share,
+                                       ModelTypeWorker* worker,
+                                       int batch_size);
+
+}  // namespace syncer
+
+#endif  // COMPONENTS_SYNC_ENGINE_IMPL_USS_MIGRATOR_H_