Add UMA metrics for ICE regathering reasons.

BUG=webrtc:6462
R=deadbeef@webrtc.org

Review URL: https://codereview.webrtc.org/2386783002 .

Cr-Commit-Position: refs/heads/master@{#14531}

webrtc/api/fakemetricsobserver.cc

@@ -45,7 +45,9 @@ void FakeMetricsObserver::AddHistogramSample(PeerConnectionMetricsName type,
 int FakeMetricsObserver::GetEnumCounter(PeerConnectionEnumCounterType type,
                                         int counter) const {
   RTC_DCHECK(thread_checker_.CalledOnValidThread());
-  RTC_CHECK(counters_.size() > static_cast<size_t>(type));
+  if (counters_.size() <= static_cast<size_t>(type)) {
+    return 0;
+  }
   const auto& it = counters_[type].find(counter);
   if (it == counters_[type].end()) {
     return 0;

webrtc/api/peerconnection.cc

@@ -1300,6 +1300,7 @@ void PeerConnection::RegisterUMAObserver(UMAObserver* observer) {
 
   // Send information about IPv4/IPv6 status.
   if (uma_observer_ && port_allocator_) {
+    port_allocator_->SetMetricsObserver(uma_observer_);
     if (port_allocator_->flags() & cricket::PORTALLOCATOR_ENABLE_IPV6) {
       uma_observer_->IncrementEnumCounter(
           kEnumCounterAddressFamily, kPeerConnection_IPv6,

webrtc/api/umametrics.h

@@ -33,6 +33,8 @@ enum PeerConnectionEnumCounterType {
   kEnumCounterDataSrtpCipher,
   kEnumCounterDataSslCipher,
   kEnumCounterDtlsHandshakeError,
+  kEnumCounterIceRegathering,
+  kEnumCounterIceRestart,
   kPeerConnectionEnumCounterMax
 };
 

webrtc/api/webrtcsession.h

@@ -293,6 +293,7 @@ class WebRtcSession :
   void set_metrics_observer(
       webrtc::MetricsObserverInterface* metrics_observer) {
     metrics_observer_ = metrics_observer;
+    transport_controller_->SetMetricsObserver(metrics_observer);
   }
 
   // Called when voice_channel_, video_channel_ and data_channel_ are created