GC observability issues with Cache API and estimate()

[wanderview]

Currently the spec requires that a Cache object continue to function even if Caches.delete() has deleted it from CacheStorage. Once the Cache js wrapper is GC'd the backing data can be removed. With Storage.estimate() it is now possible to observe when that GC takes place by polling disk usage.

This was originally noted here in a chromium issue:

https://bugs.chromium.org/p/chromium/issues/detail?id=782869

Since its a polling issue I'm not sure how critical it is. I just wanted to file this so @annevk and others are aware.