Add UMA metrics for ICE regathering reasons.

webrtc/p2p/quic/quictransportchannel.h

Index: webrtc/p2p/quic/quictransportchannel.h
diff --git a/webrtc/p2p/quic/quictransportchannel.h b/webrtc/p2p/quic/quictransportchannel.h
index 22b69b7f0f44fc996f2ceb28f02b17463b0ed145..f5a7910721f4b0b90ec56d332ed37deb0e8a179e 100644
--- a/webrtc/p2p/quic/quictransportchannel.h
+++ b/webrtc/p2p/quic/quictransportchannel.h
@@ -199,6 +199,10 @@ class QuicTransportChannel : public TransportChannelImpl,
   void OnProofVerifyDetailsAvailable(
       const net::ProofVerifyDetails& verify_details) override;
 
+  void SetMetricsObserver(webrtc::MetricsObserverInterface* observer) override {
+    channel_->SetMetricsObserver(observer);
+  }
+
   // Returns true if |quic_| has queued data which wasn't written due
   // to |channel_| being write blocked.
   bool HasDataToWrite() const;