KerberosClient: Add ClearAccounts() method

Exposes the Kerberos daemon's D-Bus method ClearAccounts(). It will be
triggered by Chrome when the kerberos.enabled flag is turned off to
remove all existing accounts. Chrome could also be done by calling
ListAccounts() and then RemoveAccount() multiple times, but that's
rather cumbersome due to the asynchronous nature of each call.

BUG=chromium:952239
TEST=None

Change-Id: I86555386fd58c90ca2644ba05d8b26c7926fe938
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1617454
Commit-Queue: Lutz Justen <ljusten@chromium.org>
Reviewed-by: Dan Erat <derat@chromium.org>
Reviewed-by: Roman Sorokin [CET] <rsorokin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#664285}
4 files changed