Add UMA metrics for ICE regathering reasons.

webrtc/p2p/base/transportcontroller.cc

Index: webrtc/p2p/base/transportcontroller.cc
diff --git a/webrtc/p2p/base/transportcontroller.cc b/webrtc/p2p/base/transportcontroller.cc
index bd6ee66871e503771b461a8ce673d319f2176ff9..1dc7ca789acd56956775b669ce9620aed0b51836 100644
--- a/webrtc/p2p/base/transportcontroller.cc
+++ b/webrtc/p2p/base/transportcontroller.cc
@@ -187,6 +187,7 @@ TransportChannel* TransportController::CreateTransportChannel_n(
   // Need to create a new channel.
   Transport* transport = GetOrCreateTransport_n(transport_name);
   TransportChannelImpl* channel = transport->CreateChannel(component);
+  channel->SetMetricsObserver(metrics_observer_);
   channel->SignalWritableState.connect(
       this, &TransportController::OnChannelWritableState_n);
   channel->SignalReceivingState.connect(
@@ -704,4 +705,12 @@ void TransportController::OnDtlsHandshakeError(rtc::SSLHandshakeError error) {
   SignalDtlsHandshakeError(error);
 }
 
+void TransportController::SetMetricsObserver(
+    webrtc::MetricsObserverInterface* metrics_observer) {
+  metrics_observer_ = metrics_observer;
+  for (auto channel : channels_) {
+    channel->SetMetricsObserver(metrics_observer);
+  }
+}
+
 }  // namespace cricket