Removes the associated FIXME in match_an_audio_or_video_type_pattern().
Sniffing process is a simplified version of the full spec, as it only
checks one frame of the mp3. To fully align with the spec, it would
also have to check a second frame by calculating frame size as
described in the spec.
Adds `matches_webm_signature()` and `parse_vint()` helpers per WPT
spec. Uses these helpers to resolve the WebM FIXME that was in
`match_an_audio_or_video_type_pattern()`.
