Allowing RTCIceServer to contain multiple URLs

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Right now RTCIceServer has two properties, url and credential, and we
recently agreed to add a third property, username, since STUN/TURN URIs no
longer contain a username.

While looking into the implementation of this change, we realized that one
will typically specify several URIs for the same TURN server, for the
different transports that can be used to access it (UDP, TCP, TLS).
However, the username/credential will be the same, regardless of transport.

So to do this using the existing RTCIceServer syntax, one must create
separate RTCIceServer objects for each URI, all with the same credentials.
It seems like it would be cleaner to allow a list of URIs to be supplied in
a RTCIceServer, which would all share the same credentials. To achieve
this, we could simply change the current .url property from a DOMString to
a list, or add a new .urls property that is a list of DOMStrings.

This is a mostly cosmetic change, but it seems like it's a better fit for
what a "RTCIceServer" really is.

Any objections?
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