style: Implement scroll-snap-align parser and serializer.

https://drafts.csswg.org/css-scroll-snap-1/#scroll-snap-align

Differential Revision: https://phabricator.services.mozilla.com/D20205

components/style/values/specified/box.rs

@@ -392,6 +392,86 @@ pub enum ScrollSnapType {
     Proximity,
 }
 
+/// Specified value of scroll-snap-align keyword value.
+#[allow(missing_docs)]
+#[derive(
+    Clone,
+    Copy,
+    Debug,
+    Eq,
+    FromPrimitive,
+    Hash,
+    MallocSizeOf,
+    Parse,
+    PartialEq,
+    SpecifiedValueInfo,
+    ToComputedValue,
+    ToCss,
+)]
+#[repr(u8)]
+pub enum ScrollSnapAlignKeyword {
+    None,
+    Start,
+    End,
+    Center,
+}
+
+/// https://drafts.csswg.org/css-scroll-snap-1/#scroll-snap-align
+#[allow(missing_docs)]
+#[cfg_attr(feature = "servo", derive(Deserialize, Serialize))]
+#[derive(
+    Clone,
+    Copy,
+    Debug,
+    Eq,
+    MallocSizeOf,
+    PartialEq,
+    SpecifiedValueInfo,
+    ToComputedValue,
+)]
+#[repr(C)]
+pub struct ScrollSnapAlign {
+    block: ScrollSnapAlignKeyword,
+    inline: ScrollSnapAlignKeyword,
+}
+
+impl ScrollSnapAlign {
+    /// Returns `none`.
+    #[inline]
+    pub fn none() -> Self {
+        ScrollSnapAlign {
+            block: ScrollSnapAlignKeyword::None,
+            inline: ScrollSnapAlignKeyword::None,
+        }
+    }
+}
+
+impl Parse for ScrollSnapAlign {
+    /// [ none | start | end | center ]{1,2}
+    fn parse<'i, 't>(
+        _context: &ParserContext,
+        input: &mut Parser<'i, 't>,
+    ) -> Result<ScrollSnapAlign, ParseError<'i>> {
+        let block = ScrollSnapAlignKeyword::parse(input)?;
+        let inline = input.try(ScrollSnapAlignKeyword::parse).unwrap_or(block);
+        Ok(ScrollSnapAlign { block, inline })
+    }
+}
+
+impl ToCss for ScrollSnapAlign {
+    fn to_css<W>(&self, dest: &mut CssWriter<W>) -> fmt::Result
+    where
+        W: Write,
+    {
+        self.block.to_css(dest)?;
+        if self.block != self.inline {
+            dest.write_str(" ")?;
+            self.inline.to_css(dest)?;
+        }
+        Ok(())
+    }
+}
+
 #[allow(missing_docs)]
 #[cfg_attr(feature = "servo", derive(Deserialize, Serialize))]
 #[derive(