Function get_optional_parameter_in_box 

fn get_optional_parameter_in_box<T: SafeFromJSValConvertible + Trace>(
    cx: &mut JSContext,
    object: HandleObject<'_>,
    parameter: &CStr,
    option: T::Config,
) -> Fallible<Option<RootedTraceableBox<T>>>

Helper to retrieve an optional paramter, in RootedTraceableBox, from WebIDL dictionary.