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Implementing this also obsoleted the updated time mechanism
for tracking whether or not our configuration is up-to-date
in CUPS.  This involved a lot of fixups to remove the last_updated
field from the Printer object.

Bug: 757595,
Change-Id: I7c75f9ce6a3d16e8bba2a9e6cf0c2e7cf7a09358
Reviewed-on: https://chromium-review.googlesource.com/627342Reviewed-by: Sean Kau <skau@chromium.org>
Commit-Queue: Justin Carlson <justincarlson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#496533}