loginDialog: Implement cancel()

The screen shield expects a cancel() method on the unlockDialog
implementation, but LoginDialog does not provide it currently.

https://bugzilla.gnome.org/show_bug.cgi?id=719378

js/gdm/loginDialog.js

@@ -885,6 +885,10 @@ const LoginDialog = new Lang.Class({
         Main.ctrlAltTabManager.removeGroup(this.dialogLayout);
     },
 
+    cancel: function() {
+        this._authPrompt.cancel();
+    },
+
     addCharacter: function(unichar) {
         this._authPrompt.addCharacter(unichar);
     },