Move, almost, all receive side references to RTP to RtpStreamReceiver.

There are still a few places in VideoReceiveStream where the RTP module
is explicitly used, e.g. setting up a/v sync, but it's a bigger task to
change and that will be done in a follow up instead of in this CL.

BUG=webrtc:5838

Review-Url: https://codereview.webrtc.org/1947913002
Cr-Commit-Position: refs/heads/master@{#12642}

webrtc/modules/rtp_rtcp/include/rtp_rtcp.h

@@ -540,6 +540,9 @@ class RtpRtcp : public Module {
 
     /*
     *   Send NACK for the packets specified.
+    *
+    *   Note: This assumes the caller keeps track of timing and doesn't rely on
+    *   the RTP module to do this.
     */
     virtual void SendNack(const std::vector<uint16_t>& sequence_numbers) = 0;