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/umametrics.h b/webrtc/api/umametrics.h
index 8dbfa22..93c034f 100644
--- a/webrtc/api/umametrics.h
+++ b/webrtc/api/umametrics.h
@@ -33,6 +33,8 @@
   kEnumCounterDataSrtpCipher,
   kEnumCounterDataSslCipher,
   kEnumCounterDtlsHandshakeError,
+  kEnumCounterIceRegathering,
+  kEnumCounterIceRestart,
   kPeerConnectionEnumCounterMax
 };