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/DEPS

diff --git a/api/DEPS b/api/DEPS
index 9756f44..c4be2a3 100644
--- a/api/DEPS
+++ b/api/DEPS
@@ -75,6 +75,10 @@
     "+rtc_base/refcount.h",
   ],
 
+  "dtlstransportinterface\.h": [
+    "+rtc_base/refcount.h",
+  ],
+
   "dtmfsenderinterface\.h": [
     "+rtc_base/refcount.h",
   ],