User Agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/75.0.3770.100 Safari/537.36

Steps to reproduce:

Please implement StorageEstimate.usageDetails

Explainer: https://jarryd999.github.io/quota-usage-details/
Spec PR: https://github.com/whatwg/storage/pull/69
Web Platform Tests (estimate-usage-details-*): https://github.com/web-platform-tests/wpt/tree/master/storage
Chrome Feature Status: https://www.chromestatus.com/feature/5737131066523648

See also discussion at https://github.com/whatwg/storage/issues/63.

(I don't there's a dedicated component for this feature, so lumping it in with localStorage for now.)

I actually have a patch that implements low level support for this:
https://phabricator.services.mozilla.com/D38028

FWIW, I think before we ship something here we better make sure all our opaque response padding is watertight.