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}
diff --git a/webrtc/api/peerconnection.cc b/webrtc/api/peerconnection.cc
index 144b1c6..7cc5d00 100644
--- a/webrtc/api/peerconnection.cc
+++ b/webrtc/api/peerconnection.cc
@@ -1300,6 +1300,7 @@
 
   // 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,