This commit begins to implement the track processing model. When the
`src` attribute is updated, we now fetch the given source file.
Currently, we always fire an `error` event once fetching is completed,
as we don't support processing the fetched data.
We have to list the set of allowed values for the DOMTokenList to not
throw when asking if one is supported.
This fixes an issue where YouTube embeds would hang indefinitely trying
to report an endless series of exceptions, seen on https://null.com/
