Bug 877474 - [music] Music app shows filename *and* title field for Ogg Vorbis files

r? @davidflanagan

I tried to keep this simple. I've tested this with Ogg Vorbis files with both single- and multiple-value fields and everything works ok. Perhaps `seen_fields` should be a Set, but I'm not sure if we should be using fancy ES6 features or not, so I erred on the side of caution.