Make SCTPtransport enter "closed" state when DTLStransport does.

Bug: webrtc:11090
Change-Id: I30e0b70387746d6c544ed1818f276569d4258cf4
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/159888
Reviewed-by: Emad Omara <emadomara@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29810}

pc/sctp_transport.h

diff --git a/pc/sctp_transport.h b/pc/sctp_transport.h
index c7727df..a13a58c 100644
--- a/pc/sctp_transport.h
+++ b/pc/sctp_transport.h
@@ -65,6 +65,8 @@
   void OnAssociationChangeCommunicationUp();
   void OnInternalClosingProcedureStartedRemotely(int sid);
   void OnInternalClosingProcedureComplete(int sid);
+  void OnDtlsStateChange(cricket::DtlsTransportInternal* transport,
+                         cricket::DtlsTransportState state);
 
   // Note - owner_thread never changes, but can't be const if we do
   // Invoke() on it.