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Adds a handler for the Kerberos Accounts settings page. Some methods
are already implemented like HandleRemoveKerberosAccount(), but others
are stubs, e.g. HandleAddKerberosAccount() does not have the UI yet to
enter credentials for a new account.

The code is largely similar to AccountManagerUIHandler. PS1 in the code
review tool contains the vanilla Account Manager code. Please diff to
PS1 for the code review.

BUG=chromium:952237
TEST=None

Change-Id: I521eac1a8aa6733a6b643971d75a555694f74e1d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1590096
Commit-Queue: Lutz Justen <ljusten@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#658477}