sync: Implement per-type update processing

sync/engine/download.h

Index: sync/engine/download.h
diff --git a/sync/engine/download.h b/sync/engine/download.h
index ddae79b115be315910f2426daaec408b12c1a5ba..7c5c5829a5a8bd3037cc6cc1482c79b194124814 100644
--- a/sync/engine/download.h
+++ b/sync/engine/download.h
@@ -55,7 +55,8 @@ SYNC_EXPORT_PRIVATE void BuildDownloadUpdatesForPoll(
 // Sends the specified message to the server and stores the response in a member
 // of the |session|'s StatusController.
 SYNC_EXPORT_PRIVATE SyncerError
-    ExecuteDownloadUpdates(sessions::SyncSession* session,
+    ExecuteDownloadUpdates(ModelTypeSet request_types,
+                           sessions::SyncSession* session,
                            sync_pb::ClientToServerMessage* msg);
 
 }  // namespace syncer