This is coming from code review https://codereview.chromium.org/2839023003/

We need to subclass AutofillManager to have a clear way to see what is used by AutofillProvider.

And we have some ideal of refactoring (copied from code review comments)

Basically, Autofill provider is only care about the interface between
AutofillManager and ContentAutofillDriver, and takes advantage of validation
code, so we can

- extract this interface and common code into an abstract class AutofillManager
- and rename current AutofillManager to AutofillManagerImpl
- Also create a new class like AutofillManagerProxy which subclass
AutofillManager also forward call to AutofillProvider,
- we create different implementation of AutofillManager in ContentAutofillDriver
based on if AutofillProvider is available.