Merge DOMSettableTokenList into DOMTokenList

[annevk]

Per discussion on IRC with @domenic and @zcorpan only HTML uses these classes. It seems like we could simplify things a bit by merging them into a single class and use [PutForward] everywhere.

The problem I could see is that libraries depend on changing the prototype of these classes in ways that are incompatible with such a simplification. What do folks think?

[zcorpan]

If changing prototypes is an issue we could do the same thing as how HTMLDocument is specced in HTML, such that window.DOMSettableTokenList === window.DOMTokenList.

[zcorpan]

In httparchive i only see http://www.gstatic.com/caja/5678/es53-taming-frame.opt.js?debug=1 containing "DOMSettableTokenList.prototype" but it was actually "TameDOMSettableTokenList.prototype".

Looking at all instances of "DOMSettableTokenList" is just a handful of copies of http://kpn.blueconic.net/frontend/static/javascript/blueconic/blueconic.min.js which does

var myGlobalVarBlacklist=["window","top","location","external","chrome", ... ,"DOMSettableTokenList"

So probably we don't need to keep window.DOMSettableTokenList around.