[webrtc-pc] Pull Request: Formalize how createOffer interacts with identity providers.

taylor-b has just submitted a new pull request for https://github.com/w3c/webrtc-pc:

== Formalize how createOffer interacts with identity providers. ==
Fixes #1184.

This cleans things up slightly, and fixes a corner case race condition,
where in the following sequence:

```
pc.setIdentityProvider(a);
pc.createOffer().then(...);
pc.setIdentityProvider(b);
pc.createOffer().then(...);
```

It wasn't completely clear if the first `createOffer` would use the
assertion from `a` or `b`. The answer now is "it will use `a`".

See https://github.com/w3c/webrtc-pc/pull/1522