[Sync] Add support for retrying a getupdates due to a context change

sync/test/engine/mock_update_handler.cc

Index: sync/test/engine/mock_update_handler.cc
diff --git a/sync/test/engine/mock_update_handler.cc b/sync/test/engine/mock_update_handler.cc
index da2db885f7b3377630fa27c397f71d37b1f2e706..b4e1bd2d9ec5fec5eeeaf304d9392104b5f891e5 100644
--- a/sync/test/engine/mock_update_handler.cc
+++ b/sync/test/engine/mock_update_handler.cc
@@ -29,12 +29,13 @@ void MockUpdateHandler::GetDataTypeContext(
   context->Clear();
 }
 
-void MockUpdateHandler::ProcessGetUpdatesResponse(
+SyncerError MockUpdateHandler::ProcessGetUpdatesResponse(
     const sync_pb::DataTypeProgressMarker& progress_marker,
     const sync_pb::DataTypeContext& mutated_context,
     const SyncEntityList& applicable_updates,
     sessions::StatusController* status) {
   progress_marker_.CopyFrom(progress_marker);
+  return syncer::SYNCER_OK;
 }
 
 void MockUpdateHandler::ApplyUpdates(sessions::StatusController* status) {