Function create_heap_buffer_source_with_length 

pub(crate) fn create_heap_buffer_source_with_length<T>(
    cx: &mut JSContext,
    len: u32,
) -> Fallible<RootedTraceableBox<HeapBufferSource<T>>>
where
    T: TypedArrayElement + TypedArrayElementCreator + 'static,
    T::Element: Clone + Copy,