Investigate CacheStorage feature for tracking usage in Tor Browser

In ESR45 we have a new CacheStorage feature that might be usable for tracking users. We should bind it to our cache isolation code if so. Even though being part of the ServiceWorker spec it is not bound to it. Quoting from comment:8:ticket:18545

The API page includes "It provides a master directory of all the named caches that a ServiceWorker, other type of worker or window scope can access (you don't have to use it with service workers, even though that is the spec that defines it) and maintains a mapping of string names to corresponding Cache objects." Also, some of the top-level objects are present in regular DOM windows. See: ​https://lists.torproject.org/pipermail/tbb-dev/2016-May/000372.html