Some audio codecs support specifying the audio encoder bitrate modes. This feature adds a "bitrateMode" flag with a default value of “variable” to WebCodec's AudioEncoderConfig, which mirrors the config option and default already present for VideoEncoderConfig. This flag will allow web authors to choose between encoding audio with a variable bitrate or a constant bitrate. Specific codec encoder implementations might have slightly different terminology (e.g. `CBR` vs `VBR` for Opus), but all of them should map to the general concept of "constant" versus "variable" bitrate. The two options have the following effects: “variable”: allows an audio encoder to increase or lower its bitrate according to the content of the audio it is encoding, in order to preserve bandwidth/binary-size, while still maintaining a target quality. For example, an encoder might lower its bitrate when encoding silence, and revert to a full bitrate when encoding speech. “constant” : forces an audio encoder to maintain the same bitrate, regardless of the audio content. This can be useful when a predictable bandwidth consumption is preferable. As of M119, this flag will affect two codecs on Chromium: Opus and AAC.