[whatwg] crossorigin property on iframe

[Ojan Vafai]

We should add a crossorigin property on iframe that causes the request to
use CORS. If it's an allowed cross-domain request, then the page should
have access to the DOM of the frame.

Also, seamless should work (assuming the CORS request succeeded of
course). One tricky thing here is that seamless needs to stop working if
the frame is navigated to a different origin to which it does not have CORS
access.

Ojan