There's no guarantee at all that the filter URI is valid, it just so happens
that for regular CSS filters we bail out earlier.
A bogus base URI makes relative uris just invalid, which triggers this assert.
The assert was gracefully handled anyway, so no big deal.
Differential Revision: https://phabricator.services.mozilla.com/D41952
--HG--
extra : moz-landing-system : lando
