Retrieving RTCRtpParameters

[aboba]

In the current editor's draft there is no way to retrieve RTCRtpParameters objects from the RTCRtpReceiver or RTCRtpSender objects. Proposed resolution is as follows:

partial interface RTCRtpSender : RTCStatsProvider {
RTCRtpParameters getParameters ();
};

partial interface RTCRtpReceiver : RTCStatsProvider {
RTCRtpParameters getParameters ();
};