Enable get() in fenced frames with network access revoked.

[VergeA]

After script running in a fenced frame successfully resolves a call to window.fence.disableUntrustedNetwork(), the fenced frame gains access to Shared Storage via get().

This patch refactors the get() algorithm to be accessible from Window and SharedStorageWorklet scopes, but the Window branch will fail outside of a fenced frame tree with network disabled.

This patch also specifies a new Permissions Policy, fenced-unpartitioned-storage-read, which can be used to disable access to get() in fenced frames. Its default allowlist is *.

[VergeA]

I think this should be ready for a first review. The spec roughly traces the path of the Chromium renderer code here, into the browser code here.

@xyaoinum, are you able to take a look at this? I don't have repo permissions to add reviewers directly. If not, I can find another reviewer.