[whatwg] Type strings to specify number of AAC audio channels

[Bolin Hsu]

Since the royalty for decoding surround sound AAC is higher than stereo
AAC, some platforms can decode stereo AAC but not surround sound AAC. But
the type string passed to HTMLMediaElement.canPlayType(type) doesn't
specify number of channels. I discussed this with some colleagues before I
found this list. Has this been covered by standard? Below are some ideas
came up during discussion:

- Use a new codec string, such as
HTMLMediaElement.canPlayType('audio/mp4; codecs="aac51"')

- Add a new parameter, such as
HTMLMediaElement.canPlayTYpe('audio/mp4; codecs="mp4a.40.2", channels="6")

- Add new element for platform configuration. Some platforms can only
output stereo audio, so the surround sound is down mixed to stereo. This
can potentially query the platform if it can really output 6 simultaneous
audio streams, amount other things:
HTMLNewElement.getCapability('audio')

Thanks,
Bolin