Reason for revert:
Unfortunately this breaks in the main waterfall: https://build.chromium.org/p/client.webrtc/builders/Android32%20Builder/builds/6362
I think it's related to dcheck_always_on=1 which is set in GYP_DEFINES only on the trybots, but not on the bots in the main waterfall.
Original issue's description:
> Implement the NackModule as part of the new jitter buffer.
>
> Things done/implemented in this CL:
> - An interface that can send Nack (VCMNackSender).
> - An interface that can request KeyFrames (VCMKeyFrameRequestSender).
> - The nack module (NackModule).
> - A set of convenience functions for modular numbers (mod_ops.h).
>
> BUG=webrtc:5514
>
> Committed: https://crrev.com/f472c5b6722dfb221f929fc4d3a2b4ca54647701
> Cr-Commit-Position: refs/heads/master@{#11882}
TBR=sprang@webrtc.org,stefan@webrtc.org,terelius@webrtc.org,torbjorng@webrtc.org,perkj@webrtc.org,tommi@webrtc.org,philipel@webrtc.org
BUG=webrtc:5514
NOTRY=True
Review URL: https://codereview.webrtc.org/1771883002
Cr-Commit-Position: refs/heads/master@{#11887}
eb648bf0e5