Revisit cross-partition redirects in prefetch

[jeremyroman]

Discussion (Google-internal, sorry) suggests that we can make this slightly less strict and allow redirects between same- and cross-partition URLs, as long as all cross-partition requests observe isolated (initially empty) storage.

Google-internal docs (sorry), but the gist is more or less the above.
https://docs.google.com/document/d/1VDo5GyoSrpntDKPLbqrTegpflsF5QDSGEkuY_IMMm6A/edit
https://docs.google.com/document/d/1Tw2lNJ8QPVnhl9fTOYniw_VdMe9KUf5MYLTW-zY8yAI/edit

[domenic]

To clarify, did we implement the changes suggested in Chromium, so this is about updating the spec to match?