Bug 271311

When we will be adding animation support for the `display` property (see bug 267762) we will need to resolve animations even when the underlying style has `display: none` to see whether animations yield a different value. In that process, we may create style-originated animations that will ultimately be canceled once we're certain `display: none` is still set. As it stands, the creation of a style-originated animation will immediately lead to animation scheduling and possibly be observable through Web content by the dispatch of animation events. As such, we must pause animation scheduling while we consider new style-originated animations.