Bitrate controller for VideoToolbox encoder.

Also fixes a crash on encoder Release.

BUG=webrtc:4081

Review URL: https://codereview.webrtc.org/1660963002

Cr-Original-Commit-Position: refs/heads/master@{#11729}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: f75d0082350096077e6553a149bda7749593b9c7
21 files changed