Add UMA metrics for ICE regathering reasons.

webrtc/p2p/base/faketransportcontroller.h

Index: webrtc/p2p/base/faketransportcontroller.h
diff --git a/webrtc/p2p/base/faketransportcontroller.h b/webrtc/p2p/base/faketransportcontroller.h
index 153aa56790a9e443dacda6a317e93c5a62b9bc92..48e6d37b81b1cdf753b7fea1dcc192c6872b7c43 100644
--- a/webrtc/p2p/base/faketransportcontroller.h
+++ b/webrtc/p2p/base/faketransportcontroller.h
@@ -293,6 +293,9 @@ class FakeTransportChannel : public TransportChannelImpl,
     return ssl_max_version_;
   }
 
+  void SetMetricsObserver(webrtc::MetricsObserverInterface* observer) override {
+  }
+
  private:
   void NegotiateSrtpCiphers() {
     for (std::vector<int>::const_iterator it1 = srtp_ciphers_.begin();