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}
diff --git a/pc/peerconnection.h b/pc/peerconnection.h
index 7e97afa..3bdd37e 100644
--- a/pc/peerconnection.h
+++ b/pc/peerconnection.h
@@ -194,6 +194,9 @@
   void SetAudioPlayout(bool playout) override;
   void SetAudioRecording(bool recording) override;
 
+  rtc::scoped_refptr<DtlsTransportInterface> LookupDtlsTransportByMid(
+      const std::string& mid) override;
+
   RTC_DEPRECATED bool StartRtcEventLog(rtc::PlatformFile file,
                                        int64_t max_size_bytes) override;
   bool StartRtcEventLog(std::unique_ptr<RtcEventLogOutput> output,