Introduce rtc::PacketTransportInterface and let cricket::TransportChannel inherit.

webrtc/p2p/base/transportcontroller.h

Index: webrtc/p2p/base/transportcontroller.h
diff --git a/webrtc/p2p/base/transportcontroller.h b/webrtc/p2p/base/transportcontroller.h
index 5f3fccc868822a1eda564345d7e2a5c7ce7b0d2e..597462b8a7cd398eddbc2cd417d4112fc481fae0 100644
--- a/webrtc/p2p/base/transportcontroller.h
+++ b/webrtc/p2p/base/transportcontroller.h
@@ -24,6 +24,7 @@
 
 namespace rtc {
 class Thread;
+struct PacketTransportInterface;
 }
 namespace webrtc {
 class MetricsObserverInterface;
@@ -204,7 +205,7 @@ class TransportController : public sigslot::has_slots<>,
   bool GetStats_n(const std::string& transport_name, TransportStats* stats);
 
   // Handlers for signals from Transport.
-  void OnChannelWritableState_n(TransportChannel* channel);
+  void OnChannelWritableState_n(rtc::PacketTransportInterface* transport);
   void OnChannelReceivingState_n(TransportChannel* channel);
   void OnChannelGatheringState_n(TransportChannelImpl* channel);
   void OnChannelCandidateGathered_n(TransportChannelImpl* channel,