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This allows the clients of UpdateClient inspect the error codes when
an update is complete.

Bug: 843657

Change-Id: I667a58a2723846cdcc4070fc9b103419ea4859d1
Reviewed-on: https://chromium-review.googlesource.com/1060586Reviewed-by: Joshua Pawlicki <waffles@chromium.org>
Commit-Queue: Sorin Jianu <sorin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#559256}