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/pc/rtc_stats_integrationtest.cc b/pc/rtc_stats_integrationtest.cc
index c140404..db49d80 100644
--- a/pc/rtc_stats_integrationtest.cc
+++ b/pc/rtc_stats_integrationtest.cc
@@ -1144,6 +1144,7 @@
     verifier.TestMemberIsPositive<uint32_t>(
         transport.selected_candidate_pair_changes);
     verifier.TestMemberIsDefined(transport.ice_role);
+    verifier.TestMemberIsDefined(transport.ice_local_username_fragment);
     return verifier.ExpectAllMembersSuccessfullyTested();
   }