incognito and guest mode stop working when using proxy

[nevoomer]

if proxy is defined, incognito and guest mode are being ignored

[mdmintz]

Duplicate of #2336 (comment)

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As mentioned in https://support.google.com/chrome_webstore/answer/2664769?hl=en -->

“Important: You can't add extensions when you browse in Incognito mode or as a guest.”

Proxy with auth requires the use of a Chrome extension to configure proxy settings. When you set SeleniumBase to use both an extension and incognito mode, it prioritizes the extension.