Implement RTP keepalive in native stack.

webrtc/modules/rtp_rtcp/source/rtp_rtcp_impl.h

Index: webrtc/modules/rtp_rtcp/source/rtp_rtcp_impl.h
diff --git a/webrtc/modules/rtp_rtcp/source/rtp_rtcp_impl.h b/webrtc/modules/rtp_rtcp/source/rtp_rtcp_impl.h
index 0c46e40e1ec38d8414679cde93de00c1ea2c4789..edec70a60a371f9ceb6a8ddbf1d51f89e7dcf646 100644
--- a/webrtc/modules/rtp_rtcp/source/rtp_rtcp_impl.h
+++ b/webrtc/modules/rtp_rtcp/source/rtp_rtcp_impl.h
@@ -335,9 +335,12 @@ class ModuleRtpRtcpImpl : public RtpRtcp, public RTCPReceiver::ModuleRtpRtcp {
   const Clock* const clock_;
 
   const bool audio_;
-  int64_t last_process_time_;
+
+  const RtpKeepAliveConfig keepalive_config_;
   int64_t last_bitrate_process_time_;
   int64_t last_rtt_process_time_;
+  int64_t next_process_time_;
+  int64_t next_keepalive_time_;
   uint16_t packet_overhead_;
 
   // Send side