Also, we put view transition psuedo element into `AnimationUtils::IsSupportedPseudoForAnimations()`, and update the handling of getting the pseudo request from a frame, to make sure we retrieve the correct data for all supported psueod elements. Besides, we have to update `SortingIndex` so we sort the events properly and avoid hitting the assertion when using view-transitions. The tests will be added when we support getAnimations() (Bug 1921109). Differential Revision: https://phabricator.services.mozilla.com/D231536