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/api/webrtcsession.h b/webrtc/api/webrtcsession.h
index c62ea99..94f146f 100644
--- a/webrtc/api/webrtcsession.h
+++ b/webrtc/api/webrtcsession.h
@@ -293,6 +293,7 @@
   void set_metrics_observer(
       webrtc::MetricsObserverInterface* metrics_observer) {
     metrics_observer_ = metrics_observer;
+    transport_controller_->SetMetricsObserver(metrics_observer);
   }
 
   // Called when voice_channel_, video_channel_ and data_channel_ are created