Add MID sending to RTPSender

This CL adds the ability to configure RTPSender to include the
MID header extension when sending packets. The MID will be
included on every packet at the start of the stream until an RTCP
acknoledgment is received for that SSRC at which point it will
stop being included. The MID will be included on regular RTP
streams as well as RTX streams.

Bug: webrtc:4050
Change-Id: Ie27ebee1cd00a67f2b931f5363788f523e3e684f
Reviewed-on: https://webrtc-review.googlesource.com/60582
Commit-Queue: Steve Anton <steveanton@webrtc.org>
Reviewed-by: Taylor Brandstetter <deadbeef@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22574}

modules/rtp_rtcp/include/rtp_rtcp.h

@@ -171,6 +171,11 @@ class RtpRtcp : public Module, public RtcpFeedbackSenderInterface {
   // Sets SSRC, default is a random number.
   virtual void SetSSRC(uint32_t ssrc) = 0;
 
+  // Sets the value for sending in the MID RTP header extension.
+  // The MID RTP header extension should be registered for this to do anything.
+  // Once set, this value can not be changed or removed.
+  virtual void SetMid(const std::string& mid) = 0;
+
   // Sets CSRC.
   // |csrcs| - vector of CSRCs
   virtual void SetCsrcs(const std::vector<uint32_t>& csrcs) = 0;