UserAgent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_2) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/71.0.3578.98 Safari/537.36

Steps to reproduce the problem:
1.  Go to chrome://flags and enable #enable-web-authentication-touch-id
2. Restart browser 
3. Initiate web authentication on a site in a normal browser window
4. Built-in sensor is offered as a means to authenticate
5. Open the same website in a private browsing window and initiate web authn.
6. Built-in sensor is no longer offered as a way to authenticate.

What is the expected behavior?
Touch ID can be used for webauthn in private browsing mode as well.

What went wrong?
Touch ID is not offered as an authentication mechanism when webauthn is initiated from a private browsing window.

Did this work before? N/A 

Chrome version: 71.0.3578.98  Channel: stable
OS Version: OS X 10.14.2
Flash Version: