When transition-behavior is allow-discrete, the animation values are
transitionable even if they are not interpoltable, given that the
animation type of the CSS property is by computed value.

Also, we remove animate() check from needs_transitions_update_per_property.
This check was added for handling the transition between auto and
other values long time ago, but now it may be redundant (because we
still pass the tests without it) and we do the same things in
nsTransitionManager as well, so it should be fine to drop it, especially
after we support discrete transitions.