PeerConnection[Interface]::GetStats(RTCStatsCollectorCallback*) added.

New file structure and targets:

rtc_stats_api
  webrtc/api/stats/rtcstats.h
  webrtc/api/stats/rtcstats_objects.h
  webrtc/api/stats/rtcstatsreport.h

rtc_stats (dep on rtc_stats_api)
  webrtc/stats/rtcstats.cc
  webrtc/stats/rtcstats_objects.cc
  webrtc/stats/rtcstatsreport.cc

libjingle_peerconnection (dep on rtc_stats)
  webrtc/api/rtcstatscollector.cc
  webrtc/api/rtcstatscollector.h

Placing rtc_stats_api headers in this separate target instead of
libjingle_peerconnection avoids a circular dependency
libjingle_peerconnection -> rtc_stats -> libjingle_peerconnection

Code changes:

PeerConnectionInterface::GetStats(RTCStatsCollectorCallback*) added for
the new stats collection API. Implemented by PeerConnection.

BUG=chromium:627816

Review-Url: https://codereview.webrtc.org/2331373004
Cr-Original-Commit-Position: refs/heads/master@{#14246}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 74e1a4f96a4915c9ad3b0ddeff87cc9797606453
21 files changed