[rtcweb] Require/Suggest AEAD GCM for SRTP

[Sean DuBois <[e-mail]>]

Hello,
I am Sean DuBois, I work on Pion [0] a 100% Go implementation of
WebRTC. I have a user that is trying to do a large deployment, and
performance is very important to them. They see a 10x performance
improvement when using AEAD GCM for SRTP (thanks to HW acceleration)
and is the most obvious improvement we can make.

Having this would be a pretty fantastic improvement for very little
work. Especially for weak devices/scaling servers, AES-NI is a huge
deal. Also great for security, avoids possible timing attacks from
software implementation and just less for developers can mess up when
implementing SRTP themselves!

This also should be pretty painless change.
* FireFox already supports it
* Chromium is just behind a flag [1]
* Most other implementations also use libsrtp (where it is already available)
* Adding more protection profiles will have zero impact if they aren't
supported.

----
I have never been involved with the IETF before, but this seems the
best way to push implementations to support it.

[0] https://github.com/pion/webrtc
[0] https://bugs.chromium.org/p/chromium/issues/detail?id=713701#c20