Replace NonNegativeLengthOrNumber by a specific type for -moz-tab-size

This is the only use of this type.

components/style/values/generics/text.rs

@@ -133,3 +133,13 @@ impl<N, L> LineHeight<N, L> {
         LineHeight::Normal
     }
 }
+
+/// A generic value for the `-moz-tab-size` property.
+#[derive(Animate, Clone, ComputeSquaredDistance, Copy, Debug, MallocSizeOf)]
+#[derive(PartialEq, ToAnimatedValue, ToAnimatedZero, ToComputedValue, ToCss)]
+pub enum MozTabSize<Number, Length> {
+    /// A number.
+    Number(Number),
+    /// A length.
+    Length(Length),
+}