In streaming mode it is preferable to fade to silence when sender stops sending, or long period of packet loss.
In streaming mode it is preferable to fade to silence when sender stops sending, or long period of packet loss.

test=try bots.
Review URL: https://webrtc-codereview.appspot.com/1272004

Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 2e6b7e938fd93d7af4e8f913d4d5243af91b87b0

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