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-Original-Commit-Position: refs/heads/master@{#16404}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: e35f89a484ca376d5c187d166714eba578dfadc3
2 files changed