Add LookupDtlsTransportByMid to proxy map

Bug: chromium:907849
Change-Id: Ib707f18fdf67617967b83cd17f0454d182b8011f
Reviewed-on: https://webrtc-review.googlesource.com/c/112907
Reviewed-by: Fredrik Solenberg <solenberg@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25885}
diff --git a/api/peerconnectioninterface.cc b/api/peerconnectioninterface.cc
index a861801..665760c9 100644
--- a/api/peerconnectioninterface.cc
+++ b/api/peerconnectioninterface.cc
@@ -178,6 +178,7 @@
 
 rtc::scoped_refptr<DtlsTransportInterface>
 PeerConnectionInterface::LookupDtlsTransportByMid(const std::string& mid) {
+  RTC_NOTREACHED();
   return nullptr;
 }