style: Switch to the new scroll-snap-type syntax for the old scroll snap implementation and drop the scroll-snap-type-{x,y} longhands.

Now scroll-snap-type is a longhand property.

Differential Revision: https://phabricator.services.mozilla.com/D21622

components/style/values/specified/box.rs

@@ -379,6 +379,32 @@ impl Parse for AnimationName {
     }
 }
 
+/// https://drafts.csswg.org/css-scroll-snap-1/#snap-axis
+#[allow(missing_docs)]
+#[cfg_attr(feature = "servo", derive(Deserialize, Serialize))]
+#[derive(
+    Clone,
+    Copy,
+    Debug,
+    Eq,
+    MallocSizeOf,
+    Parse,
+    PartialEq,
+    SpecifiedValueInfo,
+    ToComputedValue,
+    ToCss,
+    ToResolvedValue,
+    ToShmem,
+)]
+#[repr(u8)]
+pub enum ScrollSnapAxis {
+    X,
+    Y,
+    Block,
+    Inline,
+    Both,
+}
+
 /// https://drafts.csswg.org/css-scroll-snap-1/#snap-strictness
 #[allow(missing_docs)]
 #[cfg_attr(feature = "servo", derive(Deserialize, Serialize))]
@@ -398,11 +424,77 @@ impl Parse for AnimationName {
 )]
 #[repr(u8)]
 pub enum ScrollSnapStrictness {
-    None,
+    #[css(skip)]
+    None, // Used to represent scroll-snap-type: none.  It's not parsed.
     Mandatory,
     Proximity,
 }
 
+/// https://drafts.csswg.org/css-scroll-snap-1/#scroll-snap-type
+#[allow(missing_docs)]
+#[cfg_attr(feature = "servo", derive(Deserialize, Serialize))]
+#[derive(
+    Clone,
+    Copy,
+    Debug,
+    Eq,
+    MallocSizeOf,
+    PartialEq,
+    SpecifiedValueInfo,
+    ToComputedValue,
+    ToResolvedValue,
+    ToShmem,
+)]
+#[repr(C)]
+pub struct ScrollSnapType {
+    axis: ScrollSnapAxis,
+    strictness: ScrollSnapStrictness,
+}
+
+impl ScrollSnapType {
+    /// Returns `none`.
+    #[inline]
+    pub fn none() -> Self {
+        Self {
+            axis: ScrollSnapAxis::Both,
+            strictness: ScrollSnapStrictness::None,
+        }
+    }
+}
+
+impl Parse for ScrollSnapType {
+    /// none | [ x | y | block | inline | both ] [ mandatory | proximity ]?
+    fn parse<'i, 't>(
+        _context: &ParserContext,
+        input: &mut Parser<'i, 't>,
+    ) -> Result<Self, ParseError<'i>> {
+        if input.try(|input| input.expect_ident_matching("none")).is_ok() {
+            return Ok(ScrollSnapType::none());
+        }
+
+        let axis = ScrollSnapAxis::parse(input)?;
+        let strictness = input.try(ScrollSnapStrictness::parse).unwrap_or(ScrollSnapStrictness::Proximity);
+        Ok(Self { axis, strictness })
+    }
+}
+
+impl ToCss for ScrollSnapType {
+    fn to_css<W>(&self, dest: &mut CssWriter<W>) -> fmt::Result
+    where
+        W: Write,
+    {
+        if self.strictness == ScrollSnapStrictness::None {
+            return dest.write_str("none");
+        }
+        self.axis.to_css(dest)?;
+        if self.strictness != ScrollSnapStrictness::Proximity {
+            dest.write_str(" ")?;
+            self.strictness.to_css(dest)?;
+        }
+        Ok(())
+    }
+}
+
 /// Specified value of scroll-snap-align keyword value.
 #[allow(missing_docs)]
 #[derive(