Remove WeakPtrFactory from PasswordAutofillAgent

      Unlike in AutofillAgent, the factory is no longer used in PAA.

R=dvadym@chromium.org
BUG=609010,609007,608100,608101,433486

Review-Url: https://codereview.chromium.org/1945723003
Cr-Commit-Position: refs/heads/master@{#391475}