[webrtc-pc] Prefer variable ptime (audio frame lengths) (#2300)

henbos has just created a new issue for https://github.com/w3c/webrtc-pc:

== Prefer variable ptime (audio frame lengths) ==
We negotiate a maxptime (in SDP but there is no control knob for), and RTCRtpEncodingParameters.ptime allows preferring a specific ptime.

Questions:
- Today: Is an implementation allowed to vary frame lengths dynamically/autonomously during a call (e.g. depending on network conditions)?
- Do we think that enabling variation would cause backwards compatibility issues?

Possible proposal:
- Do we want an API to control whether or not we prefer varying frame lengths?

Please view or discuss this issue at https://github.com/w3c/webrtc-pc/issues/2300 using your GitHub account