DTLS-SRTP set up is bypassed when the channel has been writable.

talk/session/media/channel.h

 /*
  * libjingle
  * Copyright 2004 Google Inc.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are met:
  *
  *  1. Redistributions of source code must retain the above copyright notice,
  *     this list of conditions and the following disclaimer.
  *  2. Redistributions in binary form must reproduce the above copyright notice,
@@ skipping 161 matching lines @@
   int SetOption(SocketType type, rtc::Socket::Option o, int val)
       override;
 
   SrtpFilter* srtp_filter() { return &srtp_filter_; }
 
  protected:
   virtual MediaChannel* media_channel() const { return media_channel_; }
   // Sets the |transport_channel_| (and |rtcp_transport_channel_|, if |rtcp_| is
   // true). Gets the transport channels from |transport_controller_|.
   bool SetTransport_w(const std::string& transport_name);
+
   void set_transport_channel(TransportChannel* transport);
-  void set_rtcp_transport_channel(TransportChannel* transport);
+  void set_rtcp_transport_channel(TransportChannel* transport,
+                                  bool update_writablity);
+
   bool was_ever_writable() const { return was_ever_writable_; }
   void set_local_content_direction(MediaContentDirection direction) {
     local_content_direction_ = direction;
   }
   void set_remote_content_direction(MediaContentDirection direction) {
     remote_content_direction_ = direction;
   }
   void set_secure_required(bool secure_required) {
     secure_required_ = secure_required;
   }
@@ skipping 450 matching lines @@
   // SetSendParameters.
   DataSendParameters last_send_params_;
   // Last DataRecvParameters sent down to the media_channel() via
   // SetRecvParameters.
   DataRecvParameters last_recv_params_;
 };
 
 }  // namespace cricket
 
 #endif  // TALK_SESSION_MEDIA_CHANNEL_H_