Page Visibility IDL tests

interface Document {
};

[TreatNonObjectAsNull]
callback EventHandlerNonNull = any (Event event);
typedef EventHandlerNonNull? EventHandler;

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

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