AudioDecoderConfig.description should be `[AllowShared]`

[padenot]

The video side and chromium are [AllowShared]. This is copied, regardless, so it doesn't matter, it's just for ergonomics.

Gecko bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1958020
Chromium code: https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/renderer/modules/webcodecs/audio_decoder_config.idl;l=21

[saschanaz]

Fixed by #888.