style: Use Rust types for perspective and z-index.

Differential Revision: https://phabricator.services.mozilla.com/D20381

components/style/values/generics/position.rs

@@ -45,18 +45,22 @@ impl<H, V> Position<H, V> {
     Debug,
     MallocSizeOf,
     PartialEq,
+    Parse,
     SpecifiedValueInfo,
     ToAnimatedZero,
     ToComputedValue,
     ToCss,
 )]
-pub enum ZIndex<Integer> {
+#[repr(C, u8)]
+pub enum GenericZIndex<I> {
     /// An integer value.
-    Integer(Integer),
+    Integer(I),
     /// The keyword `auto`.
     Auto,
 }
 
+pub use self::GenericZIndex as ZIndex;
+
 impl<Integer> ZIndex<Integer> {
     /// Returns `auto`
     #[inline]