[whatwg] Support for <script crossorigin>

[Boris Zbarsky]

Both Gecko and WebKit support <script crossorigin> and allow it to 
affect whether window.onerror reports are sanitized for cross-site 
scripts.  This allows window.onerror to apply usefully to scripts from a 
CDN.

There was mention of this on this list a while ago, but the spec still 
doesn't say anything about it.  Should it?

I believe there's both an attribute and an IDL property here.

-Boris