style: Move :-moz-locale-dir matching to rust.

components/style/selector_parser.rs

@@ -183,9 +183,23 @@ pub enum Direction {
     /// right-to-left semantic directionality
     Rtl,
     /// Some other provided directionality value
+    ///
+    /// TODO(emilio): If we atomize we can then unbox in NonTSPseudoClass.
     Other(Box<str>),
 }
 
+impl Direction {
+    /// Parse a direction value.
+    pub fn parse<'i, 't>(parser: &mut CssParser<'i, 't>) -> Result<Self, ParseError<'i>> {
+        let ident = parser.expect_ident()?;
+        Ok(match_ignore_ascii_case! { &ident,
+            "rtl" => Direction::Rtl,
+            "ltr" => Direction::Ltr,
+            _ => Direction::Other(Box::from(ident.as_ref())),
+        })
+    }
+}
+
 impl ToCss for Direction {
     fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: Write {
         let dir_str = match *self {