Add UMA metrics for ICE regathering reasons.

webrtc/p2p/base/transportcontroller.h

Index: webrtc/p2p/base/transportcontroller.h
diff --git a/webrtc/p2p/base/transportcontroller.h b/webrtc/p2p/base/transportcontroller.h
index b5d9ac7d40455d53e19441e73a19a421520eced2..5f3fccc868822a1eda564345d7e2a5c7ce7b0d2e 100644
--- a/webrtc/p2p/base/transportcontroller.h
+++ b/webrtc/p2p/base/transportcontroller.h
@@ -25,6 +25,9 @@
 namespace rtc {
 class Thread;
 }
+namespace webrtc {
+class MetricsObserverInterface;
+}
 
 namespace cricket {
 
@@ -129,6 +132,8 @@ class TransportController : public sigslot::has_slots<>,
 
   sigslot::signal1<rtc::SSLHandshakeError> SignalDtlsHandshakeError;
 
+  void SetMetricsObserver(webrtc::MetricsObserverInterface* metrics_observer);
+
  protected:
   // Protected and virtual so we can override it in unit tests.
   virtual Transport* CreateTransport_n(const std::string& transport_name);
@@ -238,6 +243,8 @@ class TransportController : public sigslot::has_slots<>,
   rtc::AsyncInvoker invoker_;
   // True if QUIC is used instead of DTLS.
   bool quic_ = false;
+
+  webrtc::MetricsObserverInterface* metrics_observer_ = nullptr;
 };
 
 }  // namespace cricket