Add the last_sent_packet_id to the candidate pair change signal

webrtc/p2p/base/p2ptransportchannel.h

Index: webrtc/p2p/base/p2ptransportchannel.h
diff --git a/webrtc/p2p/base/p2ptransportchannel.h b/webrtc/p2p/base/p2ptransportchannel.h
index 23e4d7f99e507a974ea9f1286d7c812349d54bf3..4bc32e9ff60a770de36ef640216792cdf92e264a 100644
--- a/webrtc/p2p/base/p2ptransportchannel.h
+++ b/webrtc/p2p/base/p2ptransportchannel.h
@@ -193,6 +193,11 @@ class P2PTransportChannel : public TransportChannelImpl,
     return remote_candidates_;
   }
 
+  // For unit tests only.
+  void set_last_sent_packet_id(int packet_id) {
+    last_sent_packet_id_ = packet_id;
+  }
+
  private:
   rtc::Thread* thread() { return worker_thread_; }
   bool IsGettingPorts() { return allocator_session()->IsGettingPorts(); }
@@ -326,6 +331,7 @@ class P2PTransportChannel : public TransportChannelImpl,
   int weak_ping_interval_ = WEAK_PING_INTERVAL;
   TransportChannelState state_ = TransportChannelState::STATE_INIT;
   IceConfig config_;
+  int last_sent_packet_id_ = -1;  // -1 indicates no packet was sent before.
 
   RTC_DISALLOW_COPY_AND_ASSIGN(P2PTransportChannel);
 };