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-Original-Commit-Position: refs/heads/master@{#15870}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 06495bcbb7477b8df1e9f2e34eae0efd11e0b9fe
12 files changed