The point of this is to remove dependencies on privileged code from autofillEditForms.js so it can
be used in the unprivileged PaymentRequest dialog. The PaymentRequest dialog will be able to inject
the privileged APIs in a different way.
MozReview-Commit-ID: 34PvRxzWcSr
We want to re-use the field/form logic but not the dialog-specific logic so this separates them out.
Custom Elements aren't enabled by default yet so we can't use them yet.
MozReview-Commit-ID: 60hMpgSOmpp
