Bug 1087541 - Make RuleNodeWithReplacement handle animations and transitions like RulesMatching codepath does.  r=birtles

I originally wrote this to see if it would fix bug 1086937, but it
didn't.

Note that this conflicts a bit with the patch in bug 1085769; whoever
lands second will have some merging (though it shouldn't be difficult).

The updating of the style rule is needed as part of the animation-only
style update, but it shouldn't be in the general restyling code, so it
has moved there.