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*

  This was previously reverted https://codereview.webrtc.org/2465223002/
  because RTCStatsReport::Create added a new parameter not used by
  Chromium unittests. Temporarily added a default value to the argument
  to be removed after rolling and updating Chromium.

BUG=chromium:627816, chromium:657856, chromium:657854
TBR=deadbeef@webrtc.org

Review-Url: https://codereview.webrtc.org/2470703002
Cr-Original-Commit-Position: refs/heads/master@{#14866}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 6ded19086432b395a85c9a0206fa8a3bc6d33caf
10 files changed