…imation type.

The implementation is straight-forward. We have to check
if `transition-behavior` is `allow-discrete` when trying to create a new
transition and when checking if we have to cancel a running transition.

Also, the test case is out-of-date, so I tweak it a little bit and add
more general test cases for transtiion-behavior. Besides, I enable the
preference in the WPT folders which use `transition-behavior` (but
those tests may be passed already or failed due to reasons other than
`transition-behavior`).

Differential Revision: https://phabricator.services.mozilla.com/D201864

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1805727
gecko-commit: 9c80651bc3415e25aede25d358bcd0ad6e82c1de
gecko-reviewers: emilio

…o discrete.

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.

Differential Revision: https://phabricator.services.mozilla.com/D201865

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1805727
gecko-commit: 6203d25d0748a59455054f39056d4e182c4c83c9
gecko-reviewers: emilio