[USS] Add SyncMetadataStore interface

components/sync/model_impl/in_memory_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/in_memory_metadata_change_list.h"
 
 namespace syncer {
 
 InMemoryMetadataChangeList::InMemoryMetadataChangeList() {}
 InMemoryMetadataChangeList::~InMemoryMetadataChangeList() {}
@@ skipping 10 matching lines @@
 void InMemoryMetadataChangeList::UpdateMetadata(
     const std::string& storage_key,
     const sync_pb::EntityMetadata& metadata) {
   metadata_changes_[storage_key] = {UPDATE, metadata};
 }
 
 void InMemoryMetadataChangeList::ClearMetadata(const std::string& storage_key) {
   metadata_changes_[storage_key] = {CLEAR, sync_pb::EntityMetadata()};
 }
 
+base::Optional<ModelError> InMemoryMetadataChangeList::TakeError() {
+  NOTREACHED();

[pavely, 2017/04/06 22:56:42]

I don't think we should assert NOTREACHED() here. 
TakeError should either be restricted to only OnDiskMCL and therefore shouldn't
be present in MetadataChangeList and its derived classes, or it should be
allowed to call on any implementation of MetadataChangeList.

[Gang Wu, 2017/04/21 05:28:03]

Done.

+  return {};
+}
+
 }  // namespace syncer