(marking as blocking the roll until i file the trac bug and update here.)

Some more context here, the change ffmpeg made is actually okay in terms of the packets returned. The "real" issue is that discard for text streams is now broken in webm, but that's a minor issue, so I've just disabled the test for discard and linked it to this bug.

We don't really support in band text tracks, so marking this as WontFix for Chrome. If cast folks care they can look into this, but the AV_DISCARD stuff was just an optimization.