[Sync] Implement support for untracking new entities

components/sync/test/engine/mock_model_type_worker.cc

Index: components/sync/test/engine/mock_model_type_worker.cc
diff --git a/components/sync/test/engine/mock_model_type_worker.cc b/components/sync/test/engine/mock_model_type_worker.cc
index ac53569f8fcc6c5cd22cdbd1ba7ae3a923764b1e..442cf4a5e1f2a2f6b638e88a4bbaf02b928d75a9 100644
--- a/components/sync/test/engine/mock_model_type_worker.cc
+++ b/components/sync/test/engine/mock_model_type_worker.cc
@@ -118,6 +118,11 @@ void MockModelTypeWorker::UpdateFromServer(
   UpdateResponseDataList updates;
   updates.push_back(
       GenerateUpdateData(tag_hash, specifics, version_offset, ekn));
+  UpdateFromServer(updates);
+}
+
+void MockModelTypeWorker::UpdateFromServer(
+    const UpdateResponseDataList& updates) {
   processor_->OnUpdateReceived(model_type_state_, updates);
 }
 
@@ -152,6 +157,13 @@ UpdateResponseData MockModelTypeWorker::GenerateUpdateData(
   return response_data;
 }
 
+UpdateResponseData MockModelTypeWorker::GenerateUpdateData(
+    const std::string& tag_hash,
+    const sync_pb::EntitySpecifics& specifics) {
+  return GenerateUpdateData(tag_hash, specifics, 1,
+                            model_type_state_.encryption_key_name());
+}
+
 void MockModelTypeWorker::TombstoneFromServer(const std::string& tag_hash) {
   int64_t old_version = GetServerVersion(tag_hash);
   int64_t version = old_version + 1;