Support Storage Partitioning

[arichiv]

This PR is more of a find and replace right now, not all the concepts translate perfectly as https://html.spec.whatwg.org/multipage/webappapis.html#environment-settings-objects needs a storage key and https://storage.spec.whatwg.org/#storage-keys needs to define the opaque concept.

Closes #382

The following tasks have been completed:

• [X] Confirmed there are no ReSpec/BikeShed errors or warnings.
• [X] Modified Web platform tests

Implementation commitment:

• [ ] WebKit (https://bugs.webkit.org/show_bug.cgi?id=)
• [X] Chromium (https://bugs.chromium.org/p/chromium/issues/detail?id=1218100)
• [ ] Gecko (https://bugzilla.mozilla.org/show_bug.cgi?id=)

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Preview | Diff

[inexorabletash]

FYI that I made an update to https://github.com/w3c/IndexedDB/pull/334 which could use more eyeballs, and would simplify/unblock this.

[arichiv]

Could we get this in first? It seems correct as is and #334 has been in-progress for two years.