RTCIceCandidatePairStats.[state/priority] added, ConnectionInfo updated.

State and priority added to ConnectionInfo. The Connection::State enum
is replaced by IceCandidatePairState enum class.

At P2PTransportChannel::GetStats, Connection::stats is called, producing
ConnectionInfo for the connection that is then filled in with additional
values from the Connection. This is refactored so that all values are
set by Connection::stats.

RTCStatsCollector is updated to surface the ConnectionInfo stats.

BUG=webrtc:6755, chromium:633550, chromium:627816

Review-Url: https://codereview.webrtc.org/2597423003
Cr-Commit-Position: refs/heads/master@{#15870}
diff --git a/webrtc/api/rtcstats_integrationtest.cc b/webrtc/api/rtcstats_integrationtest.cc
index 577cc06..556907c 100644
--- a/webrtc/api/rtcstats_integrationtest.cc
+++ b/webrtc/api/rtcstats_integrationtest.cc
@@ -359,8 +359,8 @@
         candidate_pair.local_candidate_id, RTCLocalIceCandidateStats::kType);
     verifier.TestMemberIsIDReference(
         candidate_pair.remote_candidate_id, RTCRemoteIceCandidateStats::kType);
-    verifier.TestMemberIsUndefined(candidate_pair.state);
-    verifier.TestMemberIsUndefined(candidate_pair.priority);
+    verifier.TestMemberIsDefined(candidate_pair.state);
+    verifier.TestMemberIsNonNegative<uint64_t>(candidate_pair.priority);
     verifier.TestMemberIsUndefined(candidate_pair.nominated);
     verifier.TestMemberIsDefined(candidate_pair.writable);
     verifier.TestMemberIsUndefined(candidate_pair.readable);