[Password Autofill] Correctly save signup password forms using XHR
This mostly affects password generation as we will save the state the user
has typed. In the cases where the user hasn't typed (e.g. autofill or
generation) we have a fallback mechanism that scans forms on the page. This
check currently misses signup pages because it only looks at |password_value|
not |new_password_value|.
BUG=425653
Review URL: https://codereview.chromium.org/
672523002
Cr-Commit-Position: refs/heads/master@{#300784}