RTCCodecStats[1] added.

RTCStatsCollector supports "payloadType", "codec" and "clockRate".
"channels", "parameters" and "implementation" need to be supported
before closing crbug.com/659117.

[1] https://w3c.github.io/webrtc-stats/#codec-dict*

BUG=chromium:659117, chromium:627816, chromium:657854
NOTRY=True

Review-Url: https://codereview.webrtc.org/2509803004
Cr-Original-Commit-Position: refs/heads/master@{#15207}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 0adb8285b101ee7e8a8e1b03200018449f606d6a
5 files changed