Cache List: Bing cacheDomain is wrong.

I don't seems to be able to hit an existing host via `www.bing-amp.com`. It only works when I omit the `www` sub-domain.
aka. `bing-amp.com` returns an `OK` status whereas `www.bing-amp.com` isn't a resolvable host when I try to update the cache.

I'm not sure this is the right place to flag this. Do correct me if I'm in the wrong.

Does https://cdn.ampproject.org/caches.json really deliver a wrong cacheDomain for bing or am I missing something?

The issue mentions only https://cdn.ampproject.org/caches.json and the Bing cacheDomain entry, with no repository file or test. Start by comparing that entry with DNS behavior for bing-amp.com and www.bing-amp.com, then trace where amp-toolbox consumes the list; done means determining whether the upstream data or toolbox handling is responsible.

Written by the indexing model from the issue text.