[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 20094c7b9ac6c0a0802a1a2c901c385c59346338..0a924fd5ce54c0b3b9ad5c508947316a9a80b50d 100644
--- a/components/sync/syncable/model_type.cc
+++ b/components/sync/syncable/model_type.cc
@@ -541,6 +541,10 @@ 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