Revert "mash: Remove //ash dependency in wallpaper_private_api.cc"

This reverts commit e88248f6c5c8f2a8e0d07ea506a9a2728051e2fd.

Reason for revert: Caused bot failure/flakiness; crbug.com/835242

Original change's description:
> mash: Remove //ash dependency in wallpaper_private_api.cc
> 
> 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/1013211
> Reviewed-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}

TBR=sammc@chromium.org,xdai@chromium.org,wzang@chromium.org

Change-Id: If32a841f67435ba6066c234eca5fce41678cca1c
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 827062
Reviewed-on: https://chromium-review.googlesource.com/1021630
Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Commit-Queue: Morten Stenshorne <mstensho@chromium.org>
Cr-Commit-Position: refs/heads/master@{#552299}
13 files changed