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