iframe permissions syntax redux

As a quick note on section 15 of the PR request --

https://www.w3.org/TR/payment-request/#paymentrequest-and-iframe-elements

Before we take any steps to let the ink dry on this specification, I'd 
like to point out that Chrome appears to be using feature policy for 
these kinds of things now; cf. 
https://groups.google.com/d/msg/discuss-webrtc/32natnrbP6U/0dgpWLMkAwAJ

I know we discussed feature policy early for controlling iframe access 
early on, but there were some concerns about implementation uptake. This 
development would seem to answer at least part of that question.

-- 
Adam Roach
Principal Engineer, Mozilla