Function webkit2gtk_sys::webkit_web_view_send_message_to_page

pub unsafe extern "C" fn webkit_web_view_send_message_to_page(
    web_view: *mut WebKitWebView, 
    message: *mut WebKitUserMessage, 
    cancellable: *mut GCancellable, 
    callback: GAsyncReadyCallback, 
    user_data: gpointer
)

This is supported on crate feature v2_28 only.