Autofill for form input fields such as credit card fields, email addresses, etc. doesn't appear to work for form inputs that are inside shadow roots.
1) Go to polymer-shop2.appspot.com
2) Tap on "men's outerwear"
3) Tap on "Men's tech shell full-zip"
4) Tap on "Add to cart"
5) Tap on the cart icon on the upper right
6) Tap on "Checkout"
7) Tap in the "Email" field
The autofill selector should appear to enable you to autofill from a contact.
No autofill appears supported.
This is also the case for other fields I'd expect to autofill, like "Card Number"