Add SCTP transport to the public API.

This involves inserting an extra layer between jsep_transport_controller
and the cricket::SctpTransportInternal layer. The objects at this layer
are reference counted.

Bug: chromium:818643
Change-Id: Ibed57c4a538de981cee63e0f7f1f319f029cab39
Reviewed-on: https://webrtc-review.googlesource.com/c/123884
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Reviewed-by: Steve Anton <steveanton@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26889}
diff --git a/api/DEPS b/api/DEPS
index f5a0201..db8fdda 100644
--- a/api/DEPS
+++ b/api/DEPS
@@ -166,6 +166,10 @@
     "+rtc_base/ref_count.h",
   ],
 
+  "sctp_transport_interface\.h": [
+    "+rtc_base/ref_count.h",
+  ],
+
   "set_remote_description_observer_interface\.h": [
     "+rtc_base/ref_count.h",
   ],