The assert in question is overly conservative. The dirty rects may be
calculated using a more conservative estimate of the whole frame, and
after the animation has completed its first pass, we may have determined
that it is unnecessarily too large and shrink it accordingly. There may
still be frames lingering with the old larger rect however, and trip
this assert falsely.
Differential Revision: https://phabricator.services.mozilla.com/D113155
