Stop an SCTP connection when the DTLS transport closes.

This CL propagates a "closed" signal from DTLS up to the
SCTP section of the data channel controller, where it causes
closing of all open datachannels.

Bug: chromium:1030631, webrtc:10360
Change-Id: I88bb9e1aff5c25f330edfd092ef609d4fcc3a9f8
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/162206
Reviewed-by: Steve Anton <steveanton@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#30099}

api/transport/data_channel_transport_interface.h

@@ -84,6 +84,11 @@ class DataChannelSink {
   // invoked again following send errors (eg. due to the transport being
   // temporarily blocked or unavailable).
   virtual void OnReadyToSend() = 0;
+
+  // Callback issued when the data channel becomes unusable (closed).
+  // TODO(https://crbug.com/webrtc/10360): Make pure virtual when all
+  // consumers updated.
+  virtual void OnTransportClosed() {}
 };
 
 // Transport for data channels.