Wire up BWE stats through WebrtcSession so that they are filled in both for audio and video calls.

webrtc/media/base/mediachannel.h

Index: webrtc/media/base/mediachannel.h
diff --git a/webrtc/media/base/mediachannel.h b/webrtc/media/base/mediachannel.h
index 6f14b7aa2e5ec95c53d873feb0a6bbbf3ed56d06..6189b2f85fb2c52acc8581a5391887035ee9cd1a 100644
--- a/webrtc/media/base/mediachannel.h
+++ b/webrtc/media/base/mediachannel.h
@@ -863,13 +863,11 @@ struct VideoMediaInfo {
   void Clear() {
     senders.clear();
     receivers.clear();
-    bw_estimations.clear();
     send_codecs.clear();
     receive_codecs.clear();
   }
   std::vector<VideoSenderInfo> senders;
   std::vector<VideoReceiverInfo> receivers;
-  std::vector<BandwidthEstimationInfo> bw_estimations;
   RtpCodecParametersMap send_codecs;
   RtpCodecParametersMap receive_codecs;
 };
@@ -1083,6 +1081,7 @@ class VideoMediaChannel : public MediaChannel {
   // If SSRC is 0, the sink is used for the 'default' stream.
   virtual bool SetSink(uint32_t ssrc,
                        rtc::VideoSinkInterface<webrtc::VideoFrame>* sink) = 0;
+  virtual bool FillBitrateInfo(BandwidthEstimationInfo* bwe_info) = 0;

[Taylor Brandstetter, 2017/05/07 21:30:44]

nit: Why not "FillBandwidthEstimationInfo(BandwidthEstimationInfo*)"?

Also, what's the advantage of adding a new method over just adding
"bw_estimations" to VoiceMediaInfo? Is there a time FillBitrateInfo will be
called outside of the stats context?

[stefan-webrtc, 2017/05/08 07:12:56]

It only fills the "bitrate parts" (rtx, video bitrate) of the
BandwidthEstimationInfo, since that's the part that isn't possible to get
through Call::GetStats, since they are statistics of the send streams. I think
it would make sense to change this in the future, but I added this method now
since I don't know how we should change it, and it probably deserves a separate
CL.
I find it strange that the Call::Stats are collected via WebRtcVideoEngine2 (and
with your suggestion, WebRtcVoiceEngine). They are on a peerconnection/session
level and should only be collected once. That's why I decided to move it. I
don't think there's bw_estimations for both audio and video, there's only
bw_estimations for the currently active transport. Our current design is to have
one BWE per session.

[Taylor Brandstetter, 2017/05/08 17:47:41]

Can you describe this in a comment above FillBitrateInfo, maybe with a TODO?
I understand when you explain it that way. Can you describe this more in the CL
description?

   // Gets quality stats for the channel.
   virtual bool GetStats(VideoMediaInfo* info) = 0;
 };