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This is to support being able to set a NTP background without requiring
a network call to load collections and album photos.

Bug: 924176
Change-Id: Ie19043170e144fbf03e4b7f9dbcffcf106a2107b
Reviewed-on: https://chromium-review.googlesource.com/c/1449103
Commit-Queue: John Lee <johntlee@chromium.org>
Reviewed-by: Kyle Milka <kmilka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#628534}