[whatwg/fetch] Split up media into video, audio and track (#437)

As [discussed](http://logs.glob.uno/?c=freenode%23whatwg#c1014610) on IRC:

* `video`, `audio` and `track` types are currently a single destination: `media`
* This is because we agreed on that as part of https://github.com/whatwg/fetch/pull/211
* I agreed to this change, saying "Merging `track` into `media` is probably also possible."
* It is not. (Not in a clean way, anyway)
* `track` is quite different from `audio`/`video` and is represented internally inside Blink using a different resource type and performs different `type` checks
* @annevk prefers that if we split `track` from `media`, we'd split them all to 3 different destinations, which will enable audio/video specific `Accept` headers.
* destination is also used (and exposed) in SW, but according to @jakearchibald it is not yet shipped, so now is a good time to make this change.

/cc @igrigorik @foolip 
You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/fetch/pull/437

-- Commit Summary --

  * Split up media into video, audio and track

-- File Changes --

    M fetch.bs (6)

-- Patch Links --

https://github.com/whatwg/fetch/pull/437.patch
https://github.com/whatwg/fetch/pull/437.diff

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