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}
diff --git a/webrtc/modules/rtp_rtcp/source/rtcp_receiver.h b/webrtc/modules/rtp_rtcp/source/rtcp_receiver.h
index 2086afc..569af90 100644
--- a/webrtc/modules/rtp_rtcp/source/rtcp_receiver.h
+++ b/webrtc/modules/rtp_rtcp/source/rtcp_receiver.h
@@ -31,6 +31,7 @@
 public:
  RTCPReceiver(int32_t id,
               Clock* clock,
+              bool receiver_only,
               RtcpPacketTypeCounterObserver* packet_type_counter_observer,
               RtcpBandwidthObserver* rtcp_bandwidth_observer,
               RtcpIntraFrameObserver* rtcp_intra_frame_observer,
@@ -231,7 +232,8 @@
       uint32_t remote_ssrc, uint32_t source_ssrc) const
           EXCLUSIVE_LOCKS_REQUIRED(_criticalSectionRTCPReceiver);
 
-  Clock* _clock;
+  Clock* const _clock;
+  const bool receiver_only_;
   RTCPMethod _method;
   int64_t _lastReceived;
   ModuleRtpRtcpImpl& _rtpRtcp;