Base padding bitrate for an encoder on the bitrate allocated for that encoder, rather than the total bitrate of the channel group.

Review URL: https://codereview.webrtc.org/1231273004

Cr-Original-Commit-Position: refs/heads/master@{#9584}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: a4a8d4ad27bd0bd7a6cc5df697951d4531ddb4ee
3 files changed