Add UMA metrics for ICE regathering reasons.

webrtc/p2p/base/transportchannelimpl.h

Index: webrtc/p2p/base/transportchannelimpl.h
diff --git a/webrtc/p2p/base/transportchannelimpl.h b/webrtc/p2p/base/transportchannelimpl.h
index 0c7fa506b7d6fa1a49b5399ba0631b599523a570..1d43e54d0a8fd477e4e0ca700e20f1bcb4f7c67a 100644
--- a/webrtc/p2p/base/transportchannelimpl.h
+++ b/webrtc/p2p/base/transportchannelimpl.h
@@ -18,6 +18,10 @@
 
 namespace buzz { class XmlElement; }
 
+namespace webrtc {
+class MetricsObserverInterface;
+}
+
 namespace cricket {
 
 class Candidate;
@@ -74,6 +78,9 @@ class TransportChannelImpl : public TransportChannel {
   // occurred.
   virtual void MaybeStartGathering() = 0;
 
+  virtual void SetMetricsObserver(
+      webrtc::MetricsObserverInterface* observer) = 0;
+
   sigslot::signal1<TransportChannelImpl*> SignalGatheringState;
 
   // Handles sending and receiving of candidates.  The Transport