Don't allocate any RTPSender object for a receive only RtpRtcp module

webrtc/modules/rtp_rtcp/source/rtcp_sender.h

 /*
  *  Copyright (c) 2012 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 79 matching lines @@
   bool Sending() const;
   int32_t SetSendingStatus(const FeedbackState& feedback_state,
                            bool enabled);  // combine the functions
 
   int32_t SetNackStatus(bool enable);
 
   void SetTimestampOffset(uint32_t timestamp_offset);
 
   void SetLastRtpTime(uint32_t rtp_timestamp, int64_t capture_time_ms);
 
+  uint32_t SSRC() const;
+
   void SetSSRC(uint32_t ssrc);
 
   void SetRemoteSSRC(uint32_t ssrc);
 
   int32_t SetCNAME(const char* cName);
 
   int32_t AddMixedCNAME(uint32_t SSRC, const char* c_name);
 
   int32_t RemoveMixedCNAME(uint32_t SSRC);
 
@@ skipping 178 matching lines @@
   typedef std::unique_ptr<rtcp::RtcpPacket> (RTCPSender::*BuilderFunc)(
       const RtcpContext&);
   // Map from RTCPPacketType to builder.
   std::map<uint32_t, BuilderFunc> builders_;
 
   RTC_DISALLOW_IMPLICIT_CONSTRUCTORS(RTCPSender);
 };
 }  // namespace webrtc
 
 #endif  // WEBRTC_MODULES_RTP_RTCP_SOURCE_RTCP_SENDER_H_