Re: [w3c/manifest] Privacy Review: handle start_url tracking (#399)

It's possible the recommendation could be that UAs strip any query strong (or fragment identifier) from the URL when launching, but there are likely legit, non-privacy-invasive uses for these as well (e.g., language preference).

I guess my question would be whether this particular potential abuse vector—a dynamic `start_url`—creates a unique opportunity to gain information about a particular user that cookies, localStorage, indexed DB, and the cache API—many of which PWAs are already likely to use—don't already provide. If it does, then let's absolutely address it. If not, any mitigations we do would be relatively easy to circumvent via other means. For instance, if you want to know if the site is being viewed as a PWA or a browser tab, which would be a relatively good indication you're coming from a home screen or start menu, you can test the `display-mode` media query. And there are other APIs being discussed that might only become available if/when you are installed.

To be clear, I'm not dismissing this as a concern; it very well may be a big privacy hole. I would just like to know if the privacy concerns we identify would be unique to this particular case.

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