stats: implement iceLocalUsernameFragment

https://www.w3.org/TR/webrtc-stats/#dom-rtctransportstats-icelocalusernamefragment

BUG=webrtc:14022

Change-Id: If56ebe66d83f4e535c2245f2ca3848469914679f
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/261243
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Philipp Hancke <philipp.hancke@googlemail.com>
Cr-Commit-Position: refs/heads/main@{#36772}
diff --git a/api/stats/rtcstats_objects.h b/api/stats/rtcstats_objects.h
index 3fb68ae..373a2cd 100644
--- a/api/stats/rtcstats_objects.h
+++ b/api/stats/rtcstats_objects.h
@@ -709,6 +709,7 @@
   RTCStatsMember<std::string> srtp_cipher;
   RTCStatsMember<uint32_t> selected_candidate_pair_changes;
   RTCStatsMember<std::string> ice_role;
+  RTCStatsMember<std::string> ice_local_username_fragment;
 };
 
 }  // namespace webrtc