Adds support for hard constant bitrate (CBR) mode of the Opus encoder when CBR mode is used for MediaRecorder. Without this change it is impossible to encode compressed constant bitrate audio files with the MediaRecorder. This implements a spec change allowing either constant bitrate (CBR) or variable bitrate (VBR) encoding on the MediaRecorderOptions object passed to the MediaRecorder constructor. Previously VBR was assumed when using anything other than uncompressed PCM.
Docs: https://w3c.github.io/mediacapture-record/#mediarecorder-api
Explainers: https://docs.google.com/document/d/10OheBob6lP7v-fO89g0A5F9BYnsMEU5Nkh4p-Bk0wMg/edit?usp=sharing