[EME] Updated proposal for secure proof of key release

https://www.w3.org/Bugs/Public/show_bug.cgi?id=17199


An updated proposal is attached.

I'd like to propose that we include this text in the First Public Working Draft.

[It need not be Section 4, as written, since this interrupts the flow of the rest of the document. I leave it to David to suggest the best place for it.]

…Mark