Revert of Remove AudioTrackRenderer

Revert of Remove AudioTrackRenderer (patchset #3 id:40001 of https://codereview.webrtc.org/1399553003/ )

Reason for revert:
Breaks Chrome since its build files were not updated prior to file removal.

Original issue's description:
> - Remove AudioTrackRenderer.
> - Remove AddChannel/RemoveChannel from AudioRenderer interface.
>
> BUG=webrtc:4690
>
> Committed: https://crrev.com/1c0bb386b67835feb5934f503dddfe0912bce3ac
> Cr-Commit-Position: refs/heads/master@{#10226}

TBR=tommi@webrtc.org,solenberg@webrtc.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=webrtc:4690
Committed: https://crrev.com/eefbc3bbd7a6962265b028cf259b5028944561d1
Cr-Commit-Position: refs/heads/master@{#10228}

[torbjorng (webrtc), 2015-10-08 20:10:14]

Created Revert of Remove AudioTrackRenderer

[commit-bot: I haz the power, 2015-10-08 20:10:27]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1393343003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1393343003/1

[commit-bot: I haz the power, 2015-10-08 20:10:39]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-10-08 20:10:59]

Patchset 1 (id:??) landed as
https://crrev.com/eefbc3bbd7a6962265b028cf259b5028944561d1
Cr-Commit-Position: refs/heads/master@{#10228}