stats: implement transport iceState

https://w3c.github.io/webrtc-stats/#dom-rtctransportstats-selectedcandidatepairid

BUG=webrtc:14022

Change-Id: I206bff7048d2df3e3aff0af55072097f49d54e8f
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/261720
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Philipp Hancke <philipp.hancke@googlemail.com>
Cr-Commit-Position: refs/heads/main@{#36840}

pc/rtc_stats_integrationtest.cc

diff --git a/pc/rtc_stats_integrationtest.cc b/pc/rtc_stats_integrationtest.cc
index db49d80..4f64334 100644
--- a/pc/rtc_stats_integrationtest.cc
+++ b/pc/rtc_stats_integrationtest.cc
@@ -1145,6 +1145,7 @@
         transport.selected_candidate_pair_changes);
     verifier.TestMemberIsDefined(transport.ice_role);
     verifier.TestMemberIsDefined(transport.ice_local_username_fragment);
+    verifier.TestMemberIsDefined(transport.ice_state);
     return verifier.ExpectAllMembersSuccessfullyTested();
   }