Add UMA metrics for ICE regathering reasons.

BUG=webrtc:6462
R=deadbeef@webrtc.org

Review URL: https://codereview.webrtc.org/2386783002 .

Cr-Commit-Position: refs/heads/master@{#14531}
diff --git a/webrtc/p2p/client/basicportallocator.h b/webrtc/p2p/client/basicportallocator.h
index 3044559..d705fc5 100644
--- a/webrtc/p2p/client/basicportallocator.h
+++ b/webrtc/p2p/client/basicportallocator.h
@@ -65,6 +65,9 @@
  private:
   void Construct();
 
+  void OnIceRegathering(PortAllocatorSession* session,
+                        IceRegatheringReason reason);
+
   rtc::NetworkManager* network_manager_;
   rtc::PacketSocketFactory* socket_factory_;
   bool allow_tcp_listen_;