That comes from the first patch that followed the approach of the first LoaderClient implementation. With current implementation download signals, except decide-destination, don't need to be true_handled, and they won't have a default handler implementation. Also the download object is not supposed to be inheritable, since instances are created privately by the WebContext, so it's not possible to override the virtual methods in derived classes.
Created attachment 129274 [details]
Thanks for the patch. If this patch contains new public API please make sure it follows the guidelines for new WebKit2 GTK+ API. See http://trac.webkit.org/wiki/WebKitGTK/AddingNewWebKit2API
Committed r109117: <http://trac.webkit.org/changeset/109117>