Bitrate controller for VideoToolbox Video Encode Accelerator

This CL adds WebRTC's bitrate adjuster to Chrome's H264 HW encoder
implementation.

Design Doc: https://docs.google.com/document/d/1oUTyZdNh8QstKRds-8wHEF_hqKryMiUpEOW8M57sUGU/edit?usp=sharing

BUG=500605
TEST= Tested AppRTC loopback with Chrome flag
"--enable-webrtc-hw-h264-encoding" on
https://apprtc.appspot.com/?debug=loopback&vsc=h264

Review-Url: https://codereview.chromium.org/1818903004
Cr-Commit-Position: refs/heads/master@{#393888}
8 files changed