style: Parse '0' as a number for border-image-width.

As per CSS Values & Units:

"However, if a 0 could be parsed as either a <number> or a <length> in a
property (such as line-height), it must parse as a <number>."

(https://drafts.csswg.org/css-values-4/#lengths)

Differential Revision: https://phabricator.services.mozilla.com/D46723

components/style/values/generics/border.rs

@@ -16,6 +16,7 @@ use style_traits::{CssWriter, ToCss};
     Copy,
     Debug,
     MallocSizeOf,
+    Parse,
     PartialEq,
     SpecifiedValueInfo,
     ToComputedValue,
@@ -25,10 +26,13 @@ use style_traits::{CssWriter, ToCss};
 )]
 #[repr(C, u8)]
 pub enum GenericBorderImageSideWidth<LP, N> {
+    /// `<number>`
+    ///
+    /// NOTE: Numbers need to be before length-percentagess, in order to parse
+    /// them first, since `0` should be a number, not the `0px` length.
+    Number(N),
     /// `<length-or-percentage>`
     LengthPercentage(LP),
-    /// `<number>`
-    Number(N),
     /// `auto`
     Auto,
 }