Can't repo anymore. Will reopen if it shows up again.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/ee8bd341e805002f543357ea2b79051123eab4f3

commit ee8bd341e805002f543357ea2b79051123eab4f3
Author: Wenzhao Zang <wzang@chromium.org>
Date: Tue Jan 23 19:32:49 2018

wallpaper refactoring: Reimplement SetPolicyWallpaper

A few differences with the current implementation:

1) Currently there're both |WallpaperManager::OnPolicySet| and
   |OnPolicyFetched|. |OnPolicySet| merely labels the wallpaper
   as POLICY type to prevent potential wallpaper changes. This results
   in a temporary inconsistency between the actual wallpaper and its
   type. In this version this method is deleted. (ie, if policy fetch
   never succeeds, WallpaperController won't know about the policy.)

2) Currently |OnPolicyCleared| only sets the type back to DEFAULT
   without deleting the policy wallpaper file. This doesn't result in
   visible bugs, but it's better to delete the files and explicitly
   calls SetDefaultWallpaper for this user.

3) The existing tests in wallpaper_manager_policy_browsertest.cc stay
   unchanged in this CL. But they may be moved to a different place
   later.

Test: Verified on Admin console, no conflict with device policy.
Bug:  779221 ,  800182 
Change-Id: Ibb3fa38ddc19b095bfd684fe2fa6e500d7a580b4
Reviewed-on: https://chromium-review.googlesource.com/845262
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Commit-Queue: Wenzhao (Colin) Zang <wzang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#531301}
[modify] https://crrev.com/ee8bd341e805002f543357ea2b79051123eab4f3/ash/public/interfaces/wallpaper.mojom
[modify] https://crrev.com/ee8bd341e805002f543357ea2b79051123eab4f3/ash/wallpaper/wallpaper_controller.cc
[modify] https://crrev.com/ee8bd341e805002f543357ea2b79051123eab4f3/ash/wallpaper/wallpaper_controller.h
[modify] https://crrev.com/ee8bd341e805002f543357ea2b79051123eab4f3/ash/wallpaper/wallpaper_controller_unittest.cc
[modify] https://crrev.com/ee8bd341e805002f543357ea2b79051123eab4f3/ash/wallpaper/wallpaper_decoder.cc
[modify] https://crrev.com/ee8bd341e805002f543357ea2b79051123eab4f3/ash/wallpaper/wallpaper_decoder.h
[modify] https://crrev.com/ee8bd341e805002f543357ea2b79051123eab4f3/chrome/browser/chromeos/extensions/wallpaper_api.cc
[modify] https://crrev.com/ee8bd341e805002f543357ea2b79051123eab4f3/chrome/browser/chromeos/extensions/wallpaper_private_api.cc
[modify] https://crrev.com/ee8bd341e805002f543357ea2b79051123eab4f3/chrome/browser/chromeos/login/users/chrome_user_manager_impl.cc
[modify] https://crrev.com/ee8bd341e805002f543357ea2b79051123eab4f3/chrome/browser/chromeos/login/users/user_manager_unittest.cc
[modify] https://crrev.com/ee8bd341e805002f543357ea2b79051123eab4f3/chrome/browser/chromeos/login/users/wallpaper/wallpaper_manager.cc
[modify] https://crrev.com/ee8bd341e805002f543357ea2b79051123eab4f3/chrome/browser/chromeos/login/users/wallpaper/wallpaper_manager.h
[modify] https://crrev.com/ee8bd341e805002f543357ea2b79051123eab4f3/chrome/browser/ui/ash/test_wallpaper_controller.cc
[modify] https://crrev.com/ee8bd341e805002f543357ea2b79051123eab4f3/chrome/browser/ui/ash/test_wallpaper_controller.h
[modify] https://crrev.com/ee8bd341e805002f543357ea2b79051123eab4f3/chrome/browser/ui/ash/wallpaper_controller_client.cc
[modify] https://crrev.com/ee8bd341e805002f543357ea2b79051123eab4f3/chrome/browser/ui/ash/wallpaper_controller_client.h