<script>
    let err = '';
    function gotUserMedia(mediaStream)
    {
        window.webkit.messageHandlers.testHandler.postMessage('allowed');
    }

    function userMediaError(error)
    {
        err = `${error.name},${error.message}`;
        window.webkit.messageHandlers.testHandler.postMessage('denied');
    }

    function lastError()
    {
        return err;
    }

    let constraints = { audio: false, video: true};
    navigator.mediaDevices.getUserMedia(constraints)
        .then(gotUserMedia).
        catch(userMediaError);
</script>