style: Use Rust types for some misc properties.

-moz-tab-size, border-image-outset and border-image-slice.

This is not a particularly interesting patch, just removes some code. We can
remove way more code when a few related properties are also ported.

Differential Revision: https://phabricator.services.mozilla.com/D19825

components/style/values/generics/text.rs

@@ -149,25 +149,3 @@ impl<N, L> LineHeight<N, L> {
         LineHeight::Normal
     }
 }
-
-/// A generic value for the `-moz-tab-size` property.
-#[derive(
-    Animate,
-    Clone,
-    ComputeSquaredDistance,
-    Copy,
-    Debug,
-    MallocSizeOf,
-    PartialEq,
-    SpecifiedValueInfo,
-    ToAnimatedValue,
-    ToAnimatedZero,
-    ToComputedValue,
-    ToCss,
-)]
-pub enum MozTabSize<Number, Length> {
-    /// A number.
-    Number(Number),
-    /// A length.
-    Length(Length),
-}