js: Fix fallback dummy code chaining
js: Fix fallback dummy code chaining

If the user doesn't have a plugin, we need to make sure that the
fallback dummy methods we install to keep compatibility are able
to chain properly.

GNOME/extensions-web,magcius/sweettooth,GNOME/extensions-web,magcius/sweettooth,GNOME/extensions-web,GNOME/extensions-web