Wire up RTP keep-alive in ortc api.

[This CL is work in progress.]

Wire up the rtp keep-alive in webrtc::Call::Config using new
SetRtpTransportParameters() method on RtpTransportInterface.

BUG=webrtc:7907

Review-Url: https://codereview.webrtc.org/2981513002
Cr-Commit-Position: refs/heads/master@{#19287}

webrtc/call/rtp_transport_controller_send_interface.h

@@ -15,6 +15,7 @@ namespace webrtc {
 
 class PacketRouter;
 class RtpPacketSender;
+struct RtpKeepAliveConfig;
 class SendSideCongestionController;
 class TransportFeedbackObserver;
 
@@ -50,6 +51,7 @@ class RtpTransportControllerSendInterface {
   virtual TransportFeedbackObserver* transport_feedback_observer() = 0;
 
   virtual RtpPacketSender* packet_sender() = 0;
+  virtual const RtpKeepAliveConfig& keepalive_config() const = 0;
 };
 
 }  // namespace webrtc