style: Make offset-anchor animatable.

Per the spec issue, https://github.com/w3c/csswg-drafts/issues/3482,
we update the wpt to keep the percentage in `calc()` for `offset-anchor`.

Differential Revision: https://phabricator.services.mozilla.com/D39552

components/style/properties/longhands/box.mako.rs

@@ -427,7 +427,7 @@ ${helpers.predefined_type(
     "PositionOrAuto",
     "computed::PositionOrAuto::auto()",
     engines="gecko",
-    animation_value_type="none",
+    animation_value_type="ComputedValue",
     gecko_pref="layout.css.motion-path.enabled",
     spec="https://drafts.fxtf.org/motion-1/#offset-anchor-property",
     servo_restyle_damage="reflow_out_of_flow",

components/style/values/generics/position.rs

@@ -45,13 +45,16 @@ impl<H, V> Position<H, V> {
 /// This is used by <offset-anchor> for now.
 /// https://drafts.fxtf.org/motion-1/#offset-anchor-property
 #[derive(
+    Animate,
     Clone,
+    ComputeSquaredDistance,
     Copy,
     Debug,
     MallocSizeOf,
     Parse,
     PartialEq,
     SpecifiedValueInfo,
+    ToAnimatedZero,
     ToComputedValue,
     ToCss,
     ToResolvedValue,