stylo: Add a mechanism to restrict media-features to UA and chrome sheets.

Reviewed-by: xidorn
Bug: 1396066
MozReview-Commit-ID: 38jRV6mPbE3

components/style/parser.rs

@@ -130,6 +130,11 @@ impl<'a> ParserContext<'a> {
         };
         context.error_reporter.report_error(self.url_data, location, error)
     }
+
+    /// Returns whether this is a chrome stylesheets.
+    pub fn in_chrome_stylesheet(&self) -> bool {
+        self.url_data.is_chrome()
+    }
 }
 
 // XXXManishearth Replace all specified value parse impls with impls of this