[webrtc-pc] RTCRtpSender.getCapabilities() may not return correct information in sync (#2539)

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

== RTCRtpSender.getCapabilities() may not return correct information in sync ==
Chrome has found a [case](https://bugs.chromium.org/p/chromium/issues/detail?id=1047994) where getCapabilities() is going to have issues with delivering correct capability information about codec availability synchronously.

The two immediately obvious workarounds are:

- Fail the call with a LATER error code (but since it's a static method, there's no place to fire a "later has arrived" event at)
- Change the signature to async

The latter would be preferable from an architectural viewpoint, but has some compatibility issues.


Please view or discuss this issue at https://github.com/w3c/webrtc-pc/issues/2539 using your GitHub account