Revert "Revert of Remove SendPacer from ViEEncoder (patchset #13 id:240001 of https://codereview.webrtc.org/1917793002/ )"

This reverts commit 825eb58d59940a4c3c9837595c4b3b07059c93ca.

This Relands the cl reviewed in https://codereview.webrtc.org/1917793002/

patchset #1 is a pure reland.
patchset #2 fix an overflow in BitrateProber that caused WebRtcVideoChannel2BaseTest.TwoStreamsSendAndReceive to fail.

Original cl description:
Remove SendPacer from ViEEncoder
This CL moves the logic where the ViEEncoder pause if the pacer is full to the BitrateController. If the queue is full, the controller reports a bitrate of zero to  Call (and BitrateAllocator)

R=stefan@webrtc.org
TBR=mflodman@webrtc.org

BUG=webrtc:5687

Review URL: https://codereview.webrtc.org/1947873002 .

Cr-Commit-Position: refs/heads/master@{#12630}