legacy stats: update timestamp on localcandidate/remotecandidate

updates the timestamp on the local and remote candidate stats for consistency
with other places. This also makes the graphs on chrome://webrtc-internals
work (even though most values don't update so showing graphs is not meaningful)

BUG=chromium:937833

Change-Id: I3267dd7a5f5a887dcd0756137077b8f02c201905
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/128765
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Reviewed-by: Steve Anton <steveanton@webrtc.org>
Commit-Queue: Steve Anton <steveanton@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#27248}
diff --git a/pc/stats_collector.cc b/pc/stats_collector.cc
index 03eb59c..6bfce67 100644
--- a/pc/stats_collector.cc
+++ b/pc/stats_collector.cc
@@ -737,6 +737,7 @@
     report->AddString(StatsReport::kStatsValueNameCandidateTransportType,
                       candidate.protocol());
   }
+  report->set_timestamp(stats_gathering_started_);
 
   if (local && candidate_stats.stun_stats.has_value()) {
     const auto& stun_stats = candidate_stats.stun_stats.value();