style: Part 1: Add animation-timeline longhand property in style system

This patch adds the animation-timeline longhand property. For
shorthand, we will do that in the next patch.

This patch includes the aut-generated code in
devtools/shared/css/generated/properties-db.js, by `./mach devtools-css-db`.

Note:
1. we will use this property in Bug 1676791. For now, only make sure
   we parse it and serialize it correctly.
2. The syntax of animation-timeline may be updated, based on the spec
   issue: https://github.com/w3c/csswg-drafts/issues/6674.
   However, it's not a big problem to update it later, so we still can
   prototype this property based on the current version of spec.

Differential Revision: https://phabricator.services.mozilla.com/D126450

components/style/values/specified/box.rs

@@ -13,7 +13,7 @@ use crate::values::generics::box_::Perspective as GenericPerspective;
 use crate::values::generics::box_::{GenericVerticalAlign, VerticalAlignKeyword};
 use crate::values::specified::length::{LengthPercentage, NonNegativeLength};
 use crate::values::specified::{AllowQuirks, Number};
-use crate::values::{CustomIdent, KeyframesName};
+use crate::values::{CustomIdent, KeyframesName, TimelineName};
 use crate::Atom;
 use cssparser::Parser;
 use num_traits::FromPrimitive;
@@ -762,6 +762,62 @@ impl Parse for AnimationName {
     }
 }
 
+/// A value for the <single-animation-timeline>.
+///
+/// https://drafts.csswg.org/css-animations-2/#typedef-single-animation-timeline
+/// cbindgen:private-default-tagged-enum-constructor=false
+#[derive(
+    Clone,
+    Debug,
+    Eq,
+    Hash,
+    MallocSizeOf,
+    PartialEq,
+    SpecifiedValueInfo,
+    ToComputedValue,
+    ToCss,
+    ToResolvedValue,
+    ToShmem,
+)]
+#[repr(C, u8)]
+pub enum AnimationTimeline {
+    /// Use default timeline. The animation’s timeline is a DocumentTimeline.
+    Auto,
+    /// The animation is not associated with a timeline.
+    None,
+    /// The scroll-timeline name
+    Timeline(TimelineName),
+}
+
+impl AnimationTimeline {
+    /// Returns the `auto` value.
+    pub fn auto() -> Self {
+        Self::Auto
+    }
+}
+
+impl Parse for AnimationTimeline {
+    fn parse<'i, 't>(
+        context: &ParserContext,
+        input: &mut Parser<'i, 't>,
+    ) -> Result<Self, ParseError<'i>> {
+        // We are using the same parser for TimelineName and KeyframesName, but animation-timeline
+        // accepts "auto", so need to manually parse this. (We can not derive Parse because
+        // TimelineName excludes only "none" keyword.)
+        // FIXME: Bug 1733260: we may drop None based on the spec issue:
+        // Note: https://github.com/w3c/csswg-drafts/issues/6674.
+        if input.try_parse(|i| i.expect_ident_matching("auto")).is_ok() {
+            return Ok(Self::Auto);
+        }
+
+        if input.try_parse(|i| i.expect_ident_matching("none")).is_ok() {
+            return Ok(Self::None);
+        }
+
+        TimelineName::parse(context, input).map(AnimationTimeline::Timeline)
+    }
+}
+
 /// https://drafts.csswg.org/css-scroll-snap-1/#snap-axis
 #[allow(missing_docs)]
 #[cfg_attr(feature = "servo", derive(Deserialize, Serialize))]