Being the signal:

    webkit_gtk_page_signals[LOAD_FINISHED] = g_signal_new("load_finished",
            G_TYPE_FROM_CLASS(page_class),
            (GSignalFlags)(G_SIGNAL_RUN_LAST | G_SIGNAL_ACTION),
            NULL,
            NULL,
            NULL,
            webkit_gtk_marshal_VOID__OBJECT_BOOLEAN,
            G_TYPE_NONE, 1,
            WEBKIT_GTK_TYPE_FRAME);

The callback:

  g_signal_connect (G_OBJECT (page), "load-finished",
                    G_CALLBACK (load_finished_cb), embed);

(...)

static void
load_finished_cb (WebKitGtkPage *page,
                WebKitGtkFrame *frame,
                WebkitEmbed *wembed)

(...)

I get:

** (epiphany:17114): CRITICAL **: void webkit_gtk_marshal_VOID__OBJECT_BOOLEAN(GClosure*, GValue*, guint, const GValue*, void*, void*): assertion `n_param_values == 3' failed