sync: Introduce ModelTypeRegistry and helpers

sync/engine/syncer.cc

Index: sync/engine/syncer.cc
diff --git a/sync/engine/syncer.cc b/sync/engine/syncer.cc
index 13e1f7926184994fc9e573b90c916207969c3496..5314e62860e4bd7db6476a9d5e86c3074ef23520 100644
--- a/sync/engine/syncer.cc
+++ b/sync/engine/syncer.cc
@@ -114,11 +114,8 @@ void Syncer::ApplyUpdates(SyncSession* session) {
 
   ApplyControlDataUpdates(session->context()->directory());
 
-  UpdateHandlerMap* handler_map = session->context()->update_handler_map();
-  for (UpdateHandlerMap::iterator it = handler_map->begin();
-       it != handler_map->end(); ++it) {
-    it->second->ApplyUpdates(session->mutable_status_controller());
-  }
+  session->context()->updater_list()->ApplyUpdatesForAllTypes(
+      session->mutable_status_controller());
 
   session->context()->set_hierarchy_conflict_detected(
       session->status_controller().num_hierarchy_conflicts() > 0);
@@ -162,10 +159,11 @@ SyncerError Syncer::BuildAndPostCommits(ModelTypeSet requested_types,
     scoped_ptr<Commit> commit(
         Commit::Init(
             requested_types,
+            session->context()->enabled_types(),
             session->context()->max_commit_batch_size(),
             session->context()->account_name(),
             session->context()->directory()->cache_guid(),
-            session->context()->commit_contributor_map(),
+            session->context()->committer_list(),
             session->context()->extensions_activity()));
     if (!commit) {
       break;