This avoids redoing RTP header parsing already done in Call, for video. The next step is to convert other types of receive streams, i.e., audio and flexfec, to use a compatible OnRtpPacket method. We can then introduce a shared base interface, and simplify media-independent receive processing in Call. BUG=webrtc:7135 Review-Url: https://codereview.webrtc.org/2681673004 Cr-Commit-Position: refs/heads/master@{#16583}