Prevent sender RTCP signals for receive-only channels.

Since RTCP packets are delivered to both senders and receivers that
correspond the receivers currently log that NACKed packets are missing,
since they have no direct connection to the sending side or the RTP
packet history. Also preventing triggering on SR requests and PLI/FIR.

BUG=
R=asapersson@webrtc.org, stefan@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/45249004

Cr-Commit-Position: refs/heads/master@{#9071}

webrtc/modules/rtp_rtcp/interface/rtp_rtcp.h

@@ -54,6 +54,7 @@ class RtpRtcp : public Module {
     */
     int32_t id;
     bool audio;
+    bool receiver_only;
     Clock* clock;
     ReceiveStatistics* receive_statistics;
     Transport* outgoing_transport;