style: 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
