[Password Manager] Modified to support saving passwords on forms without user-name fields.

With current implementation forms lacking user-name fields are treated as invalid password forms and browser does not save passwords in such cases.

With this patch password manager saves passwords on forms without user-name fields as well.

BUG=406343
TBR=gcasto@chromium.org,vabr@chromium.org

Review URL: https://codereview.chromium.org/548953002

Cr-Commit-Position: refs/heads/master@{#296943}
7 files changed