Let receiving() and SignalReceivingState be part of rtc::PacketTransportInterface.

webrtc/p2p/base/packettransportinterface.h

Index: webrtc/p2p/base/packettransportinterface.h
diff --git a/webrtc/p2p/base/packettransportinterface.h b/webrtc/p2p/base/packettransportinterface.h
index c796c6c2fa11194a1391e7b0b48ba7b277dafdea..0baee3f756c9ad8cd1c188e4daa09e9586cb0f40 100644
--- a/webrtc/p2p/base/packettransportinterface.h
+++ b/webrtc/p2p/base/packettransportinterface.h
@@ -36,6 +36,9 @@ class PacketTransportInterface : public sigslot::has_slots<> {
   // The transport has been established.
   virtual bool writable() const = 0;
 
+  // The transport has received at least one packet.

[pthatcher1, 2016/10/31 18:27:30]

That's not what receiving() means.  It means that the transport has received a
packet in the last X milliseconds, where X is typically settable.  But I'm OK
with this CL without it being settable.  Perhaps update the comment to be "The
transport has received a packet in the last X milliseconds, here X is configured
by each implementation."

[johan, 2016/10/31 19:03:31]

Ok, I missed the periodic check in Connection::UpdateState /
Connection::UpdateReceiving.
Comment updated.

+  virtual bool receiving() const = 0;
+
   // Attempts to send the given packet.
   // The return value is < 0 on failure. The return value in failure case is not
   // descriptive. Depending on failure cause and implementation details
@@ -70,6 +73,9 @@ class PacketTransportInterface : public sigslot::has_slots<> {
   //  SendPacket's return code and/or GetError.
   sigslot::signal1<PacketTransportInterface*> SignalReadyToSend;
 
+  // Emitted when receiving state changes to true.
+  sigslot::signal1<PacketTransportInterface*> SignalReceivingState;
+
   // Signalled each time a packet is received on this channel.
   sigslot::signal5<PacketTransportInterface*,
                    const char*,