Add missing configuration of rtx payload type for rtp/rtcp module.

BUG=4528
R=pbos@webrtc.org, stefan@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/51639004

Cr-Commit-Position: refs/heads/master@{#8989}

webrtc/modules/rtp_rtcp/source/rtp_rtcp_impl.h

@@ -89,6 +89,7 @@ class ModuleRtpRtcpImpl : public RtpRtcp {
   void SetRtxSsrc(uint32_t ssrc) override;
 
   void SetRtxSendPayloadType(int payload_type) override;
+  int RtxSendPayloadType() const override;
 
   // Sends kRtcpByeCode when going from true to false.
   int32_t SetSendingStatus(bool sending) override;