style: Implement @supports selector() syntax.

This implements the selector(<complex-selector>) syntax for @supports.

See https://github.com/w3c/csswg-drafts/issues/3207 for explainer and
discussion.

Probably would should wait for that to be sorted out to land this, or maybe we
should put it behind a pref to get the code landed and change our
implementation if the discussion there leads to a change.

Differential Revision: https://phabricator.services.mozilla.com/D8864

components/style/stylesheets/stylesheet.rs

@@ -374,7 +374,6 @@ impl Stylesheet {
         );
 
         let rule_parser = TopLevelRuleParser {
-            stylesheet_origin: origin,
             shared_lock,
             loader: stylesheet_loader,
             context,