Updated WebRTC version to 3.31 TBR=wu@webrtc.org
Updated WebRTC version to 3.31
TBR=wu@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/1462004

Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 52b3905ec8118819a88aa617a478b9bd7ee402ef

sippet/webrtc,sippet/webrtc,sippet/webrtc,sippet/webrtc,sippet/webrtc,sippet/webrtc