Update window-properties.html to current WebIDL.

In particular, WebIDL now defines that members of the global object end up on the object itself;
see http://dev.w3.org/2006/webapi/WebIDL/#es-attributes (second bullet, second and third
sub-bullets).
Merge pull request #394 from Ms2ger/window-own-properties

Update window-properties.html to current WebIDL; r=zcorpan