Tornado secure cookie expiration (aka secure session cookie)
It seems to me that you are really on the right track. You try lower and lower values, and the cookie has a lower and lower expiration time.
Pass expires_days=None to make it a session cookie (which expires when the browser is closed).
How can I set in Tornado a secure cookie that expires when the browser is closed?
If I use set_cookie I can do this without passing extra arguments (I just set the cookie), but how if I have to use set_secure_cookie? 
I tried almost everything:

passing nothing: expiration is set to its default value, that is 1 month  
passing an integer value: the value is considered as day, i.e. 1 means 1 day  
passing a float value: it works, for example setting 0.1 it means almost one hour and a half