style: Make <ray-size> optional and default to 'closest-side'

Per the spec update, the new syntax is:
  `ray() = ray( <angle> && <ray-size>? && contain? )`
And for `<ray-size>`:
  "If no <ray-size> is specified it defaults to closest-side."

So `<ray-size>` is optional and we omit it if it's default value, for
serialization.

By the way, offset=* properties are supported only in Gecko, so we don't
need a servo function to check the preference.

Differential Revision: https://phabricator.services.mozilla.com/D171625

components/style/values/generics/motion.rs

@@ -35,6 +35,14 @@ pub enum RaySize {
     Sides,
 }
 
+impl RaySize {
+    /// Returns true if it is the default value.
+    #[inline]
+    pub fn is_default(&self) -> bool {
+        *self == RaySize::ClosestSide
+    }
+}
+
 /// The `ray()` function, `ray( [ <angle> && <size> && contain? ] )`
 ///
 /// https://drafts.fxtf.org/motion-1/#valdef-offsetpath-ray
@@ -62,6 +70,7 @@ pub struct RayFunction<Angle> {
     /// Decide the path length used when `offset-distance` is expressed
     /// as a percentage.
     #[animation(constant)]
+    #[css(skip_if = "RaySize::is_default")]
     pub size: RaySize,
     /// Clamp `offset-distance` so that the box is entirely contained
     /// within the path.

components/style/values/specified/motion.rs

@@ -16,21 +16,12 @@ use style_traits::{ParseError, StyleParseErrorKind};
 /// The specified value of `offset-path`.
 pub type OffsetPath = GenericOffsetPath<Angle>;
 
-#[cfg(feature = "gecko")]
-fn is_ray_enabled() -> bool {
-    static_prefs::pref!("layout.css.motion-path-ray.enabled")
-}
-#[cfg(feature = "servo")]
-fn is_ray_enabled() -> bool {
-    false
-}
-
 impl Parse for RayFunction<Angle> {
     fn parse<'i, 't>(
         context: &ParserContext,
         input: &mut Parser<'i, 't>,
     ) -> Result<Self, ParseError<'i>> {
-        if !is_ray_enabled() {
+        if !static_prefs::pref!("layout.css.motion-path-ray.enabled") {
             return Err(input.new_custom_error(StyleParseErrorKind::UnspecifiedError));
         }
 
@@ -60,13 +51,14 @@ impl Parse for RayFunction<Angle> {
             break;
         }
 
-        if angle.is_none() || size.is_none() {
+        if angle.is_none() {
             return Err(input.new_custom_error(StyleParseErrorKind::UnspecifiedError));
         }
 
         Ok(RayFunction {
             angle: angle.unwrap(),
-            size: size.unwrap(),
+            // If no <ray-size> is specified it defaults to closest-side.
+            size: size.unwrap_or(RaySize::ClosestSide),
             contain,
         })
     }