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/stats/rtcstats_objects.cc b/webrtc/stats/rtcstats_objects.cc
index d4fea49..487df93 100644
--- a/webrtc/stats/rtcstats_objects.cc
+++ b/webrtc/stats/rtcstats_objects.cc
@@ -19,10 +19,9 @@
 
 const char* RTCStatsIceCandidatePairState::kFrozen = "frozen";
 const char* RTCStatsIceCandidatePairState::kWaiting = "waiting";
-const char* RTCStatsIceCandidatePairState::kInProgress = "inprogress";
+const char* RTCStatsIceCandidatePairState::kInProgress = "in-progress";
 const char* RTCStatsIceCandidatePairState::kFailed = "failed";
 const char* RTCStatsIceCandidatePairState::kSucceeded = "succeeded";
-const char* RTCStatsIceCandidatePairState::kCancelled = "cancelled";
 
 // Strings defined in https://tools.ietf.org/html/rfc5245.
 const char* RTCIceCandidateType::kHost = "host";