style: Implement offset-rotate.

This includes style system and layout update. I add 3 extra reftests
because the original tests use ray() function as the offset-path, but we
don't support it. It'd be better to add tests using a different type of
offset-path.

The spec issue about the serialization:
https://github.com/w3c/fxtf-drafts/issues/340

Differential Revision: https://phabricator.services.mozilla.com/D32212

components/style/values/computed/motion.rs

@@ -4,7 +4,41 @@
 
 //! Computed types for CSS values that are related to motion path.
 
+use crate::values::computed::Angle;
+use crate::Zero;
+
 /// A computed offset-path. The computed value is as specified value.
 ///
 /// https://drafts.fxtf.org/motion-1/#offset-path-property
 pub use crate::values::specified::motion::OffsetPath;
+
+#[inline]
+fn is_auto_zero_angle(auto: &bool, angle: &Angle) -> bool {
+    *auto && angle.is_zero()
+}
+
+/// A computed offset-rotate.
+#[derive(Clone, Copy, Debug, MallocSizeOf, PartialEq, ToCss, ToResolvedValue)]
+#[repr(C)]
+pub struct OffsetRotate {
+    /// If auto is false, this is a fixed angle which indicates a
+    /// constant clockwise rotation transformation applied to it by this
+    /// specified rotation angle. Otherwise, the angle will be added to
+    /// the angle of the direction in layout.
+    #[css(represents_keyword)]
+    pub auto: bool,
+    /// The angle value.
+    #[css(contextual_skip_if = "is_auto_zero_angle")]
+    pub angle: Angle,
+}
+
+impl OffsetRotate {
+    /// Returns "auto 0deg".
+    #[inline]
+    pub fn auto() -> Self {
+        OffsetRotate {
+            auto: true,
+            angle: Zero::zero(),
+        }
+    }
+}