Implement AudioReceiveStream::GetStats().

R=tommi@webrtc.org
TBR=hta@webrtc.org
BUG=webrtc:4690

Review URL: https://codereview.webrtc.org/1390753002 .

Cr-Original-Commit-Position: refs/heads/master@{#10338}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: a457752f4afc496ed7f4d6b584b08d8635f18cc0
14 files changed