style: Simplify some conversion code used for mapped attributes.

These days for the types we share computed value representation we don't really
need any special code.

Differential Revision: https://phabricator.services.mozilla.com/D17763

components/style/values/specified/border.rs

@@ -30,6 +30,7 @@ use style_traits::{CssWriter, ParseError, ToCss};
     Copy,
     Debug,
     Eq,
+    FromPrimitive,
     MallocSizeOf,
     Ord,
     Parse,