Function webkit2gtk_sys::webkit_cookie_manager_add_cookie

pub unsafe extern "C" fn webkit_cookie_manager_add_cookie(
    cookie_manager: *mut WebKitCookieManager, 
    cookie: *mut SoupCookie, 
    cancellable: *mut GCancellable, 
    callback: GAsyncReadyCallback, 
    user_data: gpointer
)

This is supported on crate feature v2_20 only.