[USS] Add SyncMetadataStore interface

components/sync/model_impl/sync_metadata_store_change_list.cc

Index: components/sync/model_impl/sync_metadata_store_change_list.cc
diff --git a/components/sync/model_impl/sync_metadata_store_change_list.cc b/components/sync/model_impl/sync_metadata_store_change_list.cc
new file mode 100644
index 0000000000000000000000000000000000000000..59f1e89ae256f26b176b416f6b432ad6924db856
--- /dev/null
+++ b/components/sync/model_impl/sync_metadata_store_change_list.cc
@@ -0,0 +1,75 @@
+// Copyright 2017 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/sync_metadata_store_change_list.h"
+
+#include "base/location.h"
+
+using base::Optional;
+using syncer::ModelError;
+
+namespace syncer {
+
+SyncMetadataStoreChangeList::SyncMetadataStoreChangeList(
+    SyncMetadataStore* store,
+    syncer::ModelType type)
+    : store_(store), type_(type) {
+  DCHECK(store_);
+}
+
+SyncMetadataStoreChangeList::~SyncMetadataStoreChangeList() {
+  DCHECK(!error_);
+}
+
+void SyncMetadataStoreChangeList::UpdateModelTypeState(
+    const sync_pb::ModelTypeState& model_type_state) {
+  if (error_) {
+    return;
+  }
+
+  if (!store_->UpdateModelTypeState(type_, model_type_state)) {
+    error_ = ModelError(FROM_HERE, "Failed to update ModelTypeState.");
+  }
+}
+
+void SyncMetadataStoreChangeList::ClearModelTypeState() {
+  if (error_) {
+    return;
+  }
+
+  if (!store_->ClearModelTypeState(type_)) {
+    error_ = ModelError(FROM_HERE, "Failed to clear ModelTypeState.");
+  }
+}
+
+void SyncMetadataStoreChangeList::UpdateMetadata(
+    const std::string& storage_key,
+    const sync_pb::EntityMetadata& metadata) {
+  if (error_) {
+    return;
+  }
+
+  if (!store_->UpdateSyncMetadata(type_, storage_key, metadata)) {
+    error_ = ModelError(FROM_HERE, "Failed to update entity metadata.");
+  }
+}
+
+void SyncMetadataStoreChangeList::ClearMetadata(
+    const std::string& storage_key) {
+  if (error_) {
+    return;
+  }
+
+  if (!store_->ClearSyncMetadata(type_, storage_key)) {
+    error_ = ModelError(FROM_HERE, "Failed to clear entity metadata.");
+  }
+}
+
+Optional<ModelError> SyncMetadataStoreChangeList::TakeError() {
+  Optional<ModelError> temp = error_;
+  error_.reset();
+  return temp;
+}
+
+}  // namespace syncer