Reland of Enable audio streams to send padding. (patchset #4 id:60001 of https://codereview.webrtc.org/2652893004/ )

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: https://chromium.googlesource.com/external/webrtc/+/e35f89a484ca376d5c187d166714eba578dfadc3

BUG=webrtc:7043

Review-Url: https://codereview.webrtc.org/2675703002
Cr-Original-Commit-Position: refs/heads/master@{#16433}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 53b6cc3832f72db8d7aaf293723f8d2d1bef757d
9 files changed