[cssom] Add iterable<> declaration to StyleSheetList

[annevk]

Seems more useful than LegacyArrayClass (which I still think we should remove entirely if possible). (See also NodeList, which does the same.)

cc @bzbarsky @domenic @tobie

[bzbarsky]

I didn't even realize this was specced to be LegacyArrayClass. When did that happen?

Anyway, in terms of usefulness they are about equivalent. LegacyArrayClass is a bit better in terms of picking up new ES features for arrays faster (without having to wait for WebIDL updates)... But of course some UAs have no plans to implement it at all, afaict.