RTCOutboundRTPStreamStats[1] added.

This also adds RTCRTPStreamStats[2] which it derives from. Not all stats
are supported in this CL, this must be addressed before closing the
issue.

RTCStatsReport also gets a timestamp and ToString.

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

BUG=chromium:627816, chromium:657856, chromium:657854

Review-Url: https://codereview.webrtc.org/2456463002
Cr-Original-Commit-Position: refs/heads/master@{#14860}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 69e9cb08285f6cbcab547c7a5e6aa668fa6f2d29
10 files changed