Part 2: Make offset-path:path() animatable.

Here, we change the animation type of offset-path as ComputedValue, so
we could do animation on it. Also enable the wpt for offset-path
interpolation. In test_transition_per_property.html, we add some basic tests
ifor offset-path.

ToAnimatedZero for PathCommand will be dropped later.

Because the animations of arcs with mismatched flags are fallen back to
discrete animations, the result of getComputedValue is not normalized in this
case. This makes some wpt failed even though the progress is 100%.

Depends on D4786

Differential Revision: https://phabricator.services.mozilla.com/D4787

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1486094
gecko-commit: d806177c1b72aa2518294acc7b68b3e46007c626
gecko-integration-branch: autoland
gecko-reviewers: birtles

web-animations/animation-model/animation-types/property-list.js

@@ -1005,6 +1005,11 @@ const gCSSProperties = {
     types: [
     ]
   },
+  'offset-path': {
+    // https://drafts.fxtf.org/motion-1/#offset-path-property
+    types: [
+    ]
+  },
   'opacity': {
     // https://drafts.csswg.org/css-color/#propdef-opacity
     types: [