Page Visibility IDL tests

interface Document {
};
[TreatNonObjectAsNull]
callback EventHandlerNonNull = any (Event event);
typedef EventHandlerNonNull? EventHandler;

enum VisibilityState {
    "hidden",
    "visible",
    "prerender"
};

partial interface Document {
    readonly attribute boolean         hidden;
    readonly attribute VisibilityState visibilityState;
             attribute EventHandler    onvisibilitychange;
};