Original issue's description:
> Enable audio streams to send padding.
>
> Useful if bitrate probing is to be used with audio streams.
>
> BUG=webrtc:7043
>
> Review-Url: https://codereview.webrtc.org/2652893004
> Cr-Commit-Position: refs/heads/master@{#16404}
> Committed: e35f89a484
BUG=webrtc:7043
Review-Url: https://codereview.webrtc.org/2675703002
Cr-Commit-Position: refs/heads/master@{#16433}