Create the RTCRtpReceiverDelegate which is a wrapper over
webrtc::RtpReceiverObserverInterface.
The callback will be called whenever the first rtp packet is received.
Related CL: https://codereview.webrtc.org/2531333003/
BUG=webrtc:6742
Review-Url: https://codereview.webrtc.org/2641923003
Cr-Commit-Position: refs/heads/master@{#16501}