[webrtc-pc] degradationPreference is under-specified (#2248)

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

== degradationPreference is under-specified ==
`degradationPreference` appears to have been implemented differently between browsers.  While [the WPT test](https://github.com/web-platform-tests/wpt/blob/master/webrtc/RTCRtpParameters-degradationPreference.html) covers some basic aspects of its operation, it doesn't test what effect this has on the media encoding. 

Currently Section 5.2.2 says: 
"When bandwidth is constrained and the `RTCRtpSender` needs to choose between degrading resolution or degrading framerate, `degradationPreference` indicates which is preferred."

In the current Edge implementation, `degradationPreference` is interpreted as providing a content-hint to the encoder, allowing it to differentiate between applications desiring a bias toward "detail" or "motion" (in content-hints terminology).  

However understanding is that other browsers don't implement `degradationPreference` this way.

Please view or discuss this issue at https://github.com/w3c/webrtc-pc/issues/2248 using your GitHub account