[Password Generation] Always query password forms via Autofill
For other fields, we assume that if the author has specified autocomplete
attributes we should respect all their choices, including not trying to
determine field autocomplete values for fields that the author didn't specify.
This doesn't apply for password fields for two reasons. First, there is no way
for an author to specify generation for a particular field. Second, since
generation isn't a specced autocomplete value, the password field can reasonably
be set (e.g. "autocomplete=new-password") and also have generation enabled.
BUG=424793
Review URL: https://codereview.chromium.org/
659793005
Cr-Commit-Position: refs/heads/master@{#301521}