Bug
1632363 - Add support for canceling CSS transitions. r=emilio
This change adds support for canceling CSS transitions when a property
is no longer transitionable or when an element becomes styled with
display:none. Support for canceling and replacing CSS transitions when
the end value changes is still pending. This change also takes advantage
of updating the constellation message to fix a bug where transition
events could be sent for closed pipelines.
Cherry-picked from https://github.com/servo/servo/pull/26244
(though this is not part of the Gecko build).