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1) The original idea was to create a WallpaperPaths class in
   //ash/public, but it's unnecessary now. We only need a one-time
   initialization of the wallpaper paths, and they can be internal to
   //ash after that.

2) Saving and syncing thumbnail is entirely implemented in
   wallpaper_manager.js, so delete all thumbnail related code in
   *.cc files. The |thumbnail_path| variable in https://goo.gl/HZw6r7
   is left because https://chromiumcodereview.appspot.com/12334030
   forgot to clean it up.

3) Removed code duplication of thumbnail generation in wallpaper_api
   and wallpaper_private_api.

Bug: 795159
Test: Manual
Change-Id: Ia861da7579f743e5fc380128c6d28405246de2c4
Reviewed-on: https://chromium-review.googlesource.com/967882Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Reviewed-by: Will Harris <wfh@chromium.org>
Commit-Queue: Wenzhao (Colin) Zang <wzang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#546222}