[WebIDL] Allowing specifications to opt in to throwing on sets of enumerated attributes to invalid values

Right now, if you have an attribute declared with an enum type, setting 
it to a value not in the enumeration is silently ignored.  While this is 
a behavior that matches legacy XMLHttpRequest and HTML attributes, going 
forward specifications may want to throw on invalid values instead.

So I'd like to propose an extended attribute on either the IDL attribute 
or the enum itself to opt in to always throwing on invalid values.

Thoughts?

-Boris