style: Use cbindgen for border-image-width.

Differential Revision: https://phabricator.services.mozilla.com/D32032

components/style/values/generics/border.rs

@@ -23,15 +23,18 @@ use style_traits::{CssWriter, ToCss};
     ToResolvedValue,
     ToShmem,
 )]
-pub enum BorderImageSideWidth<LengthPercentage, Number> {
+#[repr(C, u8)]
+pub enum GenericBorderImageSideWidth<LP, N> {
     /// `<length-or-percentage>`
-    Length(LengthPercentage),
+    LengthPercentage(LP),
     /// `<number>`
-    Number(Number),
+    Number(N),
     /// `auto`
     Auto,
 }
 
+pub use self::GenericBorderImageSideWidth as BorderImageSideWidth;
+
 /// A generic value for the `border-image-slice` property.
 #[derive(
     Clone,