Refactor sync-specific parts out of SyncNotifier/SyncNotifierObserver

sync/notifier/invalidation_util.h

Index: sync/notifier/invalidation_util.h
diff --git a/sync/notifier/invalidation_util.h b/sync/notifier/invalidation_util.h
index bf5388f2431274bbad2ab2d045e7674995891484..c7be5fd7e17affa47a07f39fbe33d79cfba13079 100644
--- a/sync/notifier/invalidation_util.h
+++ b/sync/notifier/invalidation_util.h
@@ -29,12 +29,16 @@ struct ObjectIdLessThan {
 
 typedef std::set<invalidation::ObjectId, ObjectIdLessThan> ObjectIdSet;
 
+// TODO(dcheng): Should ModelType stuff be factored out of here?
 bool RealModelTypeToObjectId(syncer::ModelType model_type,
                              invalidation::ObjectId* object_id);
 
 bool ObjectIdToRealModelType(const invalidation::ObjectId& object_id,
                              syncer::ModelType* model_type);
 
+ObjectIdSet ModelTypeSetToObjectIdSet(const ModelTypeSet& models);
+ModelTypeSet ObjectIdSetToModelTypeSet(const ObjectIdSet& ids);
+
 std::string ObjectIdToString(const invalidation::ObjectId& object_id);
 
 std::string InvalidationToString(