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We need this because:
1) Mustash requires all //ash dependency except //ash/public to be
   removed from //chrome.
2) Currently saving and retrieving online wallpapers are handled
   separately in //chrome and //ash, and it has duplicate logic in the
   path calculation.
3) Currently wallpaper_private_api.cc is responsible for saving online
   wallpapers, so it defeats the goal of making wallpaper paths
   internal to //ash.

Also fixed a bug: on Linux build, online wallpaper is lost at login
screen.

Bug: 827062
Change-Id: I0d3ea5daf537db6e189c7a8b2ede469521e9dcbf
Reviewed-on: https://chromium-review.googlesource.com/1013211Reviewed-by: Sam McNally <sammc@chromium.org>
Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Commit-Queue: Wenzhao (Colin) Zang <wzang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#552244}