RTCCertificateStats[1] added.

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

BUG=chromium:627816, chromium:629436
NOTRY=True

Review-Url: https://codereview.webrtc.org/2243123002
Cr-Original-Commit-Position: refs/heads/master@{#14484}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 6ab97ce0b9564e31c84ea0dbf72e347499d13aa1
5 files changed