It doesn't make sense to use [MapClass] for the CacheList interface

[Boris Zbarsky]

I'm looking at 
https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#cache-list 
here.

The point of [MapClass] (assuming [MapClass] stays in the WebIDL spec at 
all) is to declare interfaces that quack enough like Map that it makes 
sense to have them be instanceof Map and have Map.prototype on their 
proto chain.

But as the "Issue" in the text notes, CacheList doesn't so much behave 
like a Map.  You put in Cache objects but get out Promise objects.  The 
set() method doesn't return the object you called it on, but something 
completely different.  has() returns a different type from ES6 Map.

I don't think it's worth trying to use MapClass here; this just isn't a 
Map in any sense, though clearly it's somewhat related to one.

-Boris