RTCDataChannelStats[1] added, supporting all stats members.

Also updates MockDataChannel to also mock id, messages_sent, bytes_sent,
messages_received and bytes_received.

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

BUG=chromium:654927, chromium:627816

Review-Url: https://codereview.webrtc.org/2420473002
Cr-Original-Commit-Position: refs/heads/master@{#14670}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: cc555c5019850f16343d0c6b00fb1abad9ba5d2a
6 files changed