[Sync] Support commit only types.

components/sync/syncable/model_type.cc

Index: components/sync/syncable/model_type.cc
diff --git a/components/sync/syncable/model_type.cc b/components/sync/syncable/model_type.cc
index e1f4d49d127d3afc456ce0b151094b3d16c65c8c..fae1e6b9eba2b1d6fe6547091d0a8c3bc27b691f 100644
--- a/components/sync/syncable/model_type.cc
+++ b/components/sync/syncable/model_type.cc
@@ -540,6 +540,10 @@ ModelTypeSet PriorityCoreTypes() {
   return result;
 }
 
+ModelTypeSet CommitOnlyTypes() {
+  return ModelTypeSet(USER_EVENTS);
+}
+
 const char* ModelTypeToString(ModelType model_type) {
   // This is used in serialization routines as well as for displaying debug
   // information.  Do not attempt to change these string values unless you know