See https://github.com/whatwg/html/issues/3740.

This header will allow top-level documents to sever window relationships when they are loaded, in cases that they have a policy that differs from the navigation initiator's top-level document.  In browsers/platforms without full Site Isolation, this can also be used to safely put such documents in a separate process without interfering with any same-origin scripting relationships.

We should implement support for this header, initially behind a flag.  This will sever window references on all Chrome platforms (switching to a new BrowsingInstance and additionally closing the previous browsing context, i.e., window).  On Android, we can also eventually use this to start treating any site serving this header on one of its documents as (dyanmically) isolated, at a site granularity.