Add API for returning a webrtc::DtlsTransport for a MID on a PC

This includes a refactoring of jseptransport to store a refcounted
object instead of a std::unique_ptr to the cricket::DtlsTransport.

Bug: chromium:907849
Change-Id: Ib557ce72c2e6ce8af297c2b8deb7ec3a103d6d31
Reviewed-on: https://webrtc-review.googlesource.com/c/111920
Reviewed-by: Fredrik Solenberg <solenberg@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25831}

api/peerconnectioninterface.cc

@@ -9,6 +9,7 @@
  */
 
 #include "api/peerconnectioninterface.h"
+#include "api/dtlstransportinterface.h"
 
 namespace webrtc {
 
@@ -175,6 +176,11 @@ bool PeerConnectionInterface::StartRtcEventLog(
   return false;
 }
 
+rtc::scoped_refptr<DtlsTransportInterface>
+PeerConnectionInterface::LookupDtlsTransportByMid(const std::string& mid) {
+  return nullptr;
+}
+
 PeerConnectionInterface::BitrateParameters::BitrateParameters() = default;
 
 PeerConnectionInterface::BitrateParameters::~BitrateParameters() = default;