[whatwg/storage] Disable prompting in third parties (#87)

In Firefox we disabled the capability for a third-party to prompt for the persistent storage permission.

Doing this helps with https://privacycg.github.io/storage-partitioning/ and also helps reduce the number of prompts where the third-party is shown.

(Delegation through the `allow=""` attribute does not work, unless we reinterpret  that as the first-party sharing its storage area, but that should probably be its own issue if there's interest in that kind of thing.)

This would be easy to add by adding a check for origin != top-level origin in the relevant places. Is there interest from Chrome?

cc @inexorabletash 

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