The Pacer now just handles interaction with Module/ProcessThread and
forwarding packets to PacketRouter.
All other logic is moved to PacedSendingController, including tests.
PacedSender unittest are now just some basic sanity tests.
Bug: webrtc:10809
Change-Id: I69223cd9d8300997375b03706d2e99c88e46241c
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/149041
Reviewed-by: Sebastian Jansson <srte@webrtc.org>
Commit-Queue: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28886}
