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/rtcstatscollector_unittest.cc b/webrtc/api/rtcstatscollector_unittest.cc
index ee09054..6e84853 100644
--- a/webrtc/api/rtcstatscollector_unittest.cc
+++ b/webrtc/api/rtcstatscollector_unittest.cc
@@ -1066,6 +1066,8 @@
   connection_info.sent_ping_requests_before_first_response = 2000;
   connection_info.recv_ping_responses = 4321;
   connection_info.sent_ping_responses = 1000;
+  connection_info.state = cricket::IceCandidatePairState::IN_PROGRESS;
+  connection_info.priority = 5555;
 
   cricket::TransportChannelStats transport_channel_stats;
   transport_channel_stats.component = cricket::ICE_CANDIDATE_COMPONENT_RTP;
@@ -1092,6 +1094,8 @@
   expected_pair.local_candidate_id = "RTCIceCandidate_" + local_candidate->id();
   expected_pair.remote_candidate_id =
       "RTCIceCandidate_" + remote_candidate->id();
+  expected_pair.state = RTCStatsIceCandidatePairState::kInProgress;
+  expected_pair.priority = 5555;
   expected_pair.writable = true;
   expected_pair.bytes_sent = 42;
   expected_pair.bytes_received = 1234;