Investigate and neuter fingerprinting potential of Permissions API

The Permissions API is now enabled in Firefox 52. We should come up with a plan to neuter its fingerprinting potential and implement it for Tor Browser 7.0:

https://bugzilla.mozilla.org/show_bug.cgi?id=1221106 https://bugzilla.mozilla.org/show_bug.cgi?id=1233702 (removes the associated pref) https://developer.mozilla.org/en-US/docs/Web/API/Permissions_API https://w3c.github.io/permissions/

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