Adding mediadiscarded event

As discussed at TPAC '14 for https://www.w3.org/Bugs/Public/show_bug.cgi?id=21877
@alvestrand ,
> what are the rules for multi-packet notifications? Do they contain data from the first packet? An exemplar packet picked at random? An aggregation of discarded packets?

This is written as though the first packet generates the event and subsequent packets (with the same SSRC) are simply discarded.  I think that is correct.  I imagine that the browser could add more information about the stream if the first packet had no header extension and later ones did, but I'm happy to leave that out from a *requirement* perspective.