style: Don't remain in an invalid state when encountering an at-rule in the wrong place.

Currently, attempting to parse an at-rule that is out of place, such as
an @import rule after a regular style rule, will cause the parser state
to be set to Invalid.  This will cause any following at-rule to be
rejected until we encounter a regular style rule, at which point we'll
go back to the Body state.  There's nothing in the CSS specs about
needing to reject all following at-rules (or, as the comment above
Invalid says, ignoring the entire rest of the style sheet).

components/style/stylesheets/stylesheet.rs

@@ -337,6 +337,7 @@ impl Stylesheet {
             loader: stylesheet_loader,
             context: context,
             state: State::Start,
+            had_hierarchy_error: false,
             namespaces: Some(namespaces),
         };