Once some or all sites are given dedicated renderer processes (using --isolate-origins or --site-per-process, respectively), the browser process can enforce that only renderer processes locked to the correct site can access that site's data.

This covers data such as cookies, passwords, localStorage, permissions, and other types of stored data.  It also includes blocking other renderer processes from receiving documents from these sites (though not all network data, since images, scripts, CSS, etc are still allowed in any web renderer process).

This is an umbrella bug to track the work to add enforcements for these properties in the browser process, so that an exploited renderer cannot access such data.