link to proof of concept?

edit: tried it on chromium 66: doesn't work.

steps to reproduce:

1. open new (non-private) tab, go to http://httpbin.org/cookies/set?k1=v1&k2=v2

2. check http://httpbin.org/cookies/, the two cookies should be there, and if you check the dev console, the expiry date is 1970-01-01 - 1 second. firefox says it's "until end of session".

3. open new private tab, go to http://httpbin.org/cookies

4. cookies from above do not show up.