Investigate fingerprinting/fpi risks for Feature Policy

Feature Policy got implemented in Firefox 64ff.

Feature Policy allows websites by different means (e.g. via the Feature-Policy header) to enable/disable plethora of features providing website owners a very fine-grained control over them. We should make sure that our first-party isolation and fingerprinting resistance is not impacted by that.

This feature is only available on nightly by default as of Firefox 73 but that might change soon.

It can be controlled by two preferences, dom.security.featurePolicy.header.enabled and dom.security.featurePolicy.webidl.enabled.