Features like localStorage, sessionStorage, and CacheStorage all work on
file:// URLs in other browsers. The spec is a bit uncertain as to when
such URLs should be treated as non-opaque, and leave it "as an exercise
to the reader".
Note that we perform this check in obtain-a-storage-key rather than in
the non-storage method, as the latter is also used for e.g. blob://
URL storage.
When we serialize blob URL entries we do not serialize the entire
environment settings object and only use it's origin. To allow
a Blob URL entry to access its relevant storage key, add a getter
that simply takes an origin.