[CHROME] Cookies can bleed to/from incognito mode

This would be an urgent bug, except for the fact that HTTPS Everywhere does not run in Chrome's incognito mode unless the user manually enables it in about:extensions.

Bram reported seeing google login state bleeding across the incognito/normal boundary.

We need to repro this, check whether it's specific to Google (Chrome has its own notion of "logged in", so that might be relevant), and fix.