Support Storage Partitioning

[arichiv]

Right now IndexedDB is partitioned by the frame's origin, but we should partition third-party iframes independently from first-party frames: https://docs.google.com/document/d/1xd6MXcUhfnZqIe5dt2CTyCn6gEZ7nOezAEWS0W9hwbQ/edit?resourcekey=0-Z5Xn1i5GJ4rit6_93QKIvA

Every time 'origin' is referenced in the spec, we should replace it with a placeholder for origin or double key'd origin depending on how the storage is partitioned.

[inexorabletash]

#334 is a start on the integration.

[arichiv]

@inexorabletash would you object to something more like w3c/ServiceWorker#1630 ? I want to make a limited change just to make it clear the storage key (instead of origin) is the way forward.

[inexorabletash]

No objection! I think when I started on the patch linked above, Storage Key wasn't exported and the intent was that storage endpoints needed to live in a bottle (or some such - it's been a while) and scoping was completely implicit. If that's changed, or we can do the work more incrementally, I'm down for reviewing PRs.