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This fixes a TODO where we weren't requesting that the server send us an
empty catalog when we have already downloaded the current version.

Bug: 898327
Change-Id: I39c0911a71947ac553ef224830d97fcdd39eed3d
Reviewed-on: https://chromium-review.googlesource.com/c/1297607Reviewed-by: Cathy Li <chili@chromium.org>
Commit-Queue: Justin DeWitt <dewittj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#602402}