Small PasswordAutofillAgent clean-ups.

1.Removing function IsElementAutocompletable.
2.Using range-based for.

Bug: 875768
Change-Id: I5d6ac8dbd554d79bce6c92480722ac4a1c14a3ee
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1752832
Commit-Queue: Vadym Doroshenko <dvadym@chromium.org>
Reviewed-by: Maxim Kolosovskiy <kolos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#688978}