Function ensure_cross_origin_property_holder 

fn ensure_cross_origin_property_holder(
    cx: JSContext,
    _proxy: HandleObject,
    cross_origin_properties: &'static CrossOriginProperties,
    out_holder: MutableHandleObject,
) -> bool

Get the holder for cross-origin properties for the current global of the JSContext, creating one and storing it in a slot of the proxy object if it doesn’t exist yet.

This essentially creates a cache of CrossOriginGetOwnPropertyHelper’s results for all property keys.

cx and proxy are expected to be different-Realm here. proxy is a proxy for a maybe-cross-origin object. The out_holder return value will always be in the Realm of cx.