With this patch, we now enforce basic same-origin policy for this one
<iframe> attribute.
To make it easier to add more attributes like this, I've added an
extended IDL attribute ("[ReturnNullIfCrossOrigin]") that does exactly
what it sounds like. :^)
Note that these aren't full implementations of the bindings. This
mostly implements the low hanging fruit (namely, basic reflections)
There are some attributes that should be USVString instead of
DOMString. However, USVString is a slightly different definition
of DOMString, so it should suffice for now.
