The buggy animation handling isn't a regression, since currently we pass
UnstyledChildrenOnly in those cases, which blocks the animation traversal
in Servo_TraverseSubtree.
In general I really wanted to handle these two paths together. But there's
enough broken with the NewChildren path that I wanted to scope the buginess
as tightly as possible. And I really need to separate the handling here from
StyleDocument() to make the restyle root stuff work.
MozReview-Commit-ID: 9F0mcQl7AAX
