Constant CROSS_ORIGIN_PROPERTY_HOLDER_WEAK_MAP_SLOT 

pub const CROSS_ORIGIN_PROPERTY_HOLDER_WEAK_MAP_SLOT: u32 = 1; // 1u32

In WindowProxy and ProxyObject, the second slot is reserved for the cross-origin property holder weak map. This is a weak map between the realm and a property holder which stores objects that are used in cross origin realms.