style: Introduce css(parse_condition).

This will allow us to add a pref for this, and to parse it only on chrome easily.

Bug: 1288572
Reviewed-by: xidorn
MozReview-Commit-ID: L1rsyc2A2hu

components/style_derive/to_css.rs

@@ -235,6 +235,7 @@ pub struct CssVariantAttrs {
     pub dimension: bool,
     pub keyword: Option<String>,
     pub aliases: Option<String>,
+    pub parse_condition: Option<Path>,
     pub skip: bool,
 }