Add UMA metrics for ICE regathering reasons.

webrtc/p2p/base/dtlstransportchannel.h

Index: webrtc/p2p/base/dtlstransportchannel.h
diff --git a/webrtc/p2p/base/dtlstransportchannel.h b/webrtc/p2p/base/dtlstransportchannel.h
index 19823f8c346d9c922b1f77fce1e614464b2f34ee..4cc75b18fe12dfabe54354c69970672c0d95fa92 100644
--- a/webrtc/p2p/base/dtlstransportchannel.h
+++ b/webrtc/p2p/base/dtlstransportchannel.h
@@ -186,6 +186,10 @@ class DtlsTransportChannelWrapper : public TransportChannelImpl {
     channel_->RemoveRemoteCandidate(candidate);
   }
 
+  void SetMetricsObserver(webrtc::MetricsObserverInterface* observer) override {
+    channel_->SetMetricsObserver(observer);
+  }
+
   void SetIceConfig(const IceConfig& config) override {
     channel_->SetIceConfig(config);
   }