[Sync] USS: Implement SimpleMetadataChangeList.

sync/internal_api/public/simple_metadata_change_list.cc

 // Copyright 2015 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 "sync/internal_api/public/simple_metadata_change_list.h"
 
 namespace syncer_v2 {
 
 SimpleMetadataChangeList::SimpleMetadataChangeList() {}
 
 SimpleMetadataChangeList::~SimpleMetadataChangeList() {}
 
 void SimpleMetadataChangeList::UpdateDataTypeState(
     const sync_pb::DataTypeState& data_type_state) {
-  // TODO(skym): Implementation.
+  state_change_.reset(new DataTypeStateChange{UPDATE, data_type_state});
+}
+
+void SimpleMetadataChangeList::ClearDataTypeState() {
+  state_change_.reset(new DataTypeStateChange{CLEAR});
 }
 
 void SimpleMetadataChangeList::UpdateMetadata(
     const std::string& client_tag,
     const sync_pb::EntityMetadata& metadata) {
-  // TODO(skym): Implementation.
-}
-
-void SimpleMetadataChangeList::ClearDataTypeState() {
-  // TODO(skym): Implementation.
+  metadata_changes_[client_tag] = {UPDATE, metadata};
 }
 
 void SimpleMetadataChangeList::ClearMetadata(const std::string& client_tag) {
-  // TODO(skym): Implementation.
+  metadata_changes_[client_tag] = {CLEAR, sync_pb::EntityMetadata()};
 }
 
-void SimpleMetadataChangeList::TranfserChanges(
+const SimpleMetadataChangeList::MetadataChanges&
+SimpleMetadataChangeList::GetMetadataChanges() const {
+  return metadata_changes_;
+}
+
+bool SimpleMetadataChangeList::HasDataTypeStateChange() const {
+  return state_change_.get() != nullptr;
+}
+
+const SimpleMetadataChangeList::DataTypeStateChange&
+SimpleMetadataChangeList::GetDataTypeStateChange() const {
+  return *state_change_.get();
+}
+
+void SimpleMetadataChangeList::TransferChanges(
     ModelTypeStore* store,
     ModelTypeStore::WriteBatch* write_batch) {
   // TODO(skym): Implementation.
 }
 
 }  // namespace syncer_v2