[css3-animations] OM for keyframes

Implementing an editor for animations, I discover an API on keyframes
is missing. If we have |findRule()| to find the keyframe rule
corresponding to a given key, there is no API to retrieve the list
of keys defined by a keyframes at-rule... And that's something
editing tools will all do.

I would recommend adding such a list; something like

   readonly attribute DOMStringList keyList;

added to CSSKeyframesRule. It would contain an ordered list of
unique keys, from 'from' (or 0%) to 'to' (or 100%) in increasing %age
order.

Thoughts?

</Daniel>