Add support for RTCTransportStats.selectedCandidatePairChanges

This patch adds accounting and reporting needed for
newly added RTCTransportStats.selectedCandidatePairChanges,
https://w3c.github.io/webrtc-stats/#dom-rtctransportstats-selectedcandidatepairchanges

a) P2PTransportChannel counts everytime selected_connection_
is modified and reports this counter in the GetStats()-call.
b) RTCStatsCollector puts the counter into the standardized
stats object.

Bug: webrtc:10900
Change-Id: Ibaeca18706b8edcbcb44b0c6f2754854bcb545ba
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/149830
Reviewed-by: Qingsi Wang <qingsi@webrtc.org>
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Commit-Queue: Jonas Oreland <jonaso@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28987}

pc/jsep_transport.cc

diff --git a/pc/jsep_transport.cc b/pc/jsep_transport.cc
index 519c6fc..c800232 100644
--- a/pc/jsep_transport.cc
+++ b/pc/jsep_transport.cc
@@ -747,7 +747,7 @@
   dtls_transport->GetSslCipherSuite(&substats.ssl_cipher_suite);
   substats.dtls_state = dtls_transport->dtls_state();
   if (!dtls_transport->ice_transport()->GetStats(
-          &substats.connection_infos, &substats.candidate_stats_list)) {
+          &substats.ice_transport_stats)) {
     return false;
   }
   stats->channel_stats.push_back(substats);