[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..e499392805889cf72f76628e944b4f8ae12e5eb4 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(
+bool 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 true;
 }
 
 void MockUpdateHandler::ApplyUpdates(sessions::StatusController* status) {