We were handling the STYLE tag getting added in the
dom_default_action_DOMSubtreeModified_cb callback, however,
the media attribute dadn't been added by that point.
We now check it in the dom_default_action_DOMNodeInserted_cb
callback, at which time the media attribute is available for
checking.
Note, we should probably store the media type in the
html_stylesheet structure, but for now we just have an
`unused` boolean.
Also, it may be possible to optimise to avoid parsing non-
screen media STYLE element data if we could detect the media
earlier.