Simplify CongestionController.

- Removes the dependency on CallStats.
- Implements Module interface so that we don't have to register
  each internal component to the process thread separately.

R=pbos@webrtc.org, solenberg@webrtc.org

Review URL: https://codereview.webrtc.org/1704983002 .

Cr-Original-Commit-Position: refs/heads/master@{#11655}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 789ba92e1462c4ad45dc13bec6616658df30c663
10 files changed