[preload-tests] [preload] Split `media` into `video`, `audio` and `track` (#5033)

This changes `as` values of `media` to their corresponding appropriate destination as defined in https://github.com/whatwg/fetch/pull/437

/cc @annevk 

View on GitHub: https://github.com/w3c/web-platform-tests/pull/5033

Received on Wednesday, 1 March 2017 07:21:00 UTC