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/border.rs

@@ -26,7 +26,8 @@ pub enum BorderImageSideWidth<LengthPercentage, Number> {
 #[derive(
     Clone, Copy, Debug, MallocSizeOf, PartialEq, SpecifiedValueInfo, ToComputedValue, ToCss,
 )]
-pub struct BorderImageSlice<NumberOrPercentage> {
+#[repr(C)]
+pub struct GenericBorderImageSlice<NumberOrPercentage> {
     /// The offsets.
     #[css(field_bound)]
     pub offsets: Rect<NumberOrPercentage>,
@@ -35,6 +36,8 @@ pub struct BorderImageSlice<NumberOrPercentage> {
     pub fill: bool,
 }
 
+pub use self::GenericBorderImageSlice as BorderImageSlice;
+
 /// A generic value for the `border-*-radius` longhand properties.
 #[derive(
     Animate,