QoS 0 still accesses localStorage

[This comment][1] suggests that using QoS 0 would prevent the use of `localStorage`.

> if the network connection or browser fails. If you do not wish for the Local Storage to be used, please do not use this client, or only use QoS 0 to send and receive messages.

However, when using QoS 0 I still receive an exception about access to localStorage being denied in Chrome; it works in Firefox.

> Uncaught DOMException: Failed to read the 'localStorage' property from 'Window': Access is denied for this document.

I am loading my code into an iframe on another webpage on a separate domain which perhaps has something to do with it. That said however, it'd be useful if using QoS 0 actually prevented the use of localStorage as it's not necessary. Looking at [the code here][2], that's not the case.

It seems that this PR https://github.com/eclipse/paho.mqtt.javascript/pull/191 would indirectly fix my issue as the Exception would no longer crash my application, but doesn't address the fact that QoS 0 doesn't disable localStorage usage.

[1]: https://github.com/eclipse/paho.mqtt.javascript/blob/master/utility/index.html#L62
[2]: https://github.com/eclipse/paho.mqtt.javascript/blob/413f72920575f699f895ad1016283c053da9d4f0/src/paho-mqtt.js#L109