[Sync] Adding a passthrough MetadataChangeList implementation.

components/sync/model_impl/passthrough_metadata_change_list.cc

+// 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.
+
+#include "components/sync/model_impl/passthrough_metadata_change_list.h"
+
+#include "base/logging.h"
+#include "components/sync/protocol/entity_metadata.pb.h"
+#include "components/sync/protocol/model_type_state.pb.h"
+
+namespace syncer {
+
+PassthroughMetadataChangeList::PassthroughMetadataChangeList(
+    ModelTypeStore* store,
+    ModelTypeStore::WriteBatch* batch)
+    : store_(store), batch_(batch) {
+  DCHECK(store_);
+  DCHECK(batch_);
+}
+
+void PassthroughMetadataChangeList::UpdateModelTypeState(
+    const sync_pb::ModelTypeState& model_type_state) {
+  store_->WriteGlobalMetadata(batch_, model_type_state.SerializeAsString());
+}
+
+void PassthroughMetadataChangeList::ClearModelTypeState() {
+  store_->DeleteGlobalMetadata(batch_);
+}
+
+void PassthroughMetadataChangeList::UpdateMetadata(
+    const std::string& storage_key,
+    const sync_pb::EntityMetadata& metadata) {
+  store_->WriteMetadata(batch_, storage_key, metadata.SerializeAsString());
+}
+
+void PassthroughMetadataChangeList::ClearMetadata(
+    const std::string& storage_key) {
+  store_->DeleteMetadata(batch_, storage_key);
+}
+
+}  // namespace syncer