Initial support for auto-filling fields in iframes

Collect input fields for possible autofill from nested
iframes as well as the main page.

Currently only same-origin iframes are supported.
Supporting sites which embed a login form in an iframe with
a different-but-related domain (eg. https://connect.garmin.com
hosts a login form from sso.garmin.com) will require
additional work due to same-origin restrictions in <iframe>
access in the browser.