[w3c/FileAPI] Blob URL store partitioning (#153)

https://privacycg.github.io/storage-partitioning/ has some general background here and https://trac.torproject.org/projects/tor/ticket/15502 is much more specific.

@bakulf was thinking that we could restrict blob URL lookup to the agent cluster (in addition to origin, that is). The one tweak I would suggest to that is that navigating a top-level browsing context (including a noopener one) to a blob URL still ought to work.

Concretely, this would mean that if you have `https://example.com/` open twice, in separate browsing context groups, any blob URLs they mint cannot be used by the other.

The one gotcha with the tweak I suggested is that the other could observe existence through a popup then. Now that's an attack that's unlikely to yield anything useful in practice, but we could break that too by forcing noopener or a version of COOP that never matches (and thus always creates a new browsing context group).

We suspect this to be web-compatible and are happy to try it out in Firefox.

cc @mkruisselbrink @hober @SubhamoyS 

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