RTCMediaStreamTrackStats.kind added and collected.

Implements recent spec change:
https://github.com/w3c/webrtc-stats/pull/127

BUG=chromium:659137, chromium:627816

Review-Url: https://codereview.webrtc.org/2623513006
Cr-Original-Commit-Position: refs/heads/master@{#16111}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 160e4a78e30041258a86e115bddaf5074f36d2b6
5 files changed