The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/9f63cbc3e1ef27826818187cb7dc982bbd7d686f

commit 9f63cbc3e1ef27826818187cb7dc982bbd7d686f
Author: Wenzhao Zang <wzang@chromium.org>
Date: Fri May 11 20:31:10 2018

cros: Fix preview wallpaper reloading issue

In |ReloadWallpaper|, the |current_wallpaper_| should be reset, however
if the current wallpaper is the preview wallpaper, it's not available
from |ShowUserWallpaper| yet because the preview wallpaper hasn't been
set for the user. So we need some other ways to retrieve the image.

Although we can remove the reset of |current_wallpaper_| for the
preview case, in that way we lose the ability to resize the preview
wallpaper based on the new display size (|current_wallpaper_| stores
the image after resizing, but we need the original image to go through
the resizing process again in order to show an accurate preview.)

So creating another callback should be a better solution.

Bug:  835349 
Change-Id: I6a76646eacb99dae26867303647206d316275950
Reviewed-on: https://chromium-review.googlesource.com/1052686
Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Commit-Queue: Wenzhao (Colin) Zang <wzang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#558006}
[modify] https://crrev.com/9f63cbc3e1ef27826818187cb7dc982bbd7d686f/ash/wallpaper/wallpaper_controller.cc
[modify] https://crrev.com/9f63cbc3e1ef27826818187cb7dc982bbd7d686f/ash/wallpaper/wallpaper_controller.h
[modify] https://crrev.com/9f63cbc3e1ef27826818187cb7dc982bbd7d686f/ash/wallpaper/wallpaper_controller_unittest.cc