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

talk/session/media/channel.h

Index: talk/session/media/channel.h
diff --git a/talk/session/media/channel.h b/talk/session/media/channel.h
index 8faefe6d7b570c69185410237f04d68db28a5b8b..3a8550cfae3e424ade9f6b3449d957c8909d6c13 100644
--- a/talk/session/media/channel.h
+++ b/talk/session/media/channel.h
@@ -179,8 +179,11 @@ class BaseChannel
   // 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;