Convert minimp3 to platform decoder

[classilla]

Regression from 38, but MP3 playback goes through a different pathway we should try to leverage in 45 and we should just convert minimp3 to that.

[classilla]

It doesn't (fully), but it does help, and it fixes other glitches.

[classilla]

The new Amazon code drop seems to have completely broken us in other ways. No idea why.

[classilla]

Still works fine in 38.

[classilla]

Also occurs in 40.0.3, so that narrows the regression range. We'll need a tool to actually profile the browser to figure out where it spends its time (Shark dies on symbol resolution), so let's just narrow this bug to the platform decoder (which is working).

[classilla]

Shipp'd (the platform decoder)