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Implements the UI and Chrome backend aspects of remembering the last
password for each Kerberos account. The password is stored by the
Kerberos daemon and never leaves it. Chrome just sends an empty password
if it wants the daemon to reuse the stored one.

BUG=chromium:952243
TEST=Turn KerberosEnabled policy on, add account in the Kerberos
     settings page, click 'Remember password' checkbox, click the 'Add'
     button. Next time you sign in (there's no way to enforce that
     right now, so you might have to wait a while) the password should
     be preset and the 'Add' button should be focused. If you overwrite
     the password, the new password should be taken.

Change-Id: If2486659cbf87216a2f8dd4a4362e39935a7f47e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1634863
Commit-Queue: Lutz Justen <ljusten@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Mattias Nissler <mnissler@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: Dan Erat <derat@chromium.org>
Cr-Commit-Position: refs/heads/master@{#668606}