The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/3da560edaced9383d3dd33f71d6263eadf9444b0

commit 3da560edaced9383d3dd33f71d6263eadf9444b0
Author: xdai <xdai@chromium.org>
Date: Wed Dec 20 22:01:03 2017

wallpaper refactoring: Implement WallpaperPolicyHandler.

This CL is part of "Move SetDeviceWallpaperIfApplicable() to WallpaperController in ash" effort.
It's not functional yet since I haven't hooked up the new WallpaperPolicyHandler to
WallpaperControllerClient. I will hook them up and also clean up the old approach in
WallpaperManager in a following CL.

Bug:  791654 
Change-Id: I821dcdcfe2f193ee71d9f33295516cd6034c7791
Reviewed-on: https://chromium-review.googlesource.com/820854
Commit-Queue: Xiaoqian Dai <xdai@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#525484}
[modify] https://crrev.com/3da560edaced9383d3dd33f71d6263eadf9444b0/ash/public/interfaces/wallpaper.mojom
[modify] https://crrev.com/3da560edaced9383d3dd33f71d6263eadf9444b0/ash/wallpaper/wallpaper_controller.cc
[modify] https://crrev.com/3da560edaced9383d3dd33f71d6263eadf9444b0/ash/wallpaper/wallpaper_controller.h
[modify] https://crrev.com/3da560edaced9383d3dd33f71d6263eadf9444b0/chrome/browser/ui/BUILD.gn
[modify] https://crrev.com/3da560edaced9383d3dd33f71d6263eadf9444b0/chrome/browser/ui/ash/test_wallpaper_controller.cc
[modify] https://crrev.com/3da560edaced9383d3dd33f71d6263eadf9444b0/chrome/browser/ui/ash/test_wallpaper_controller.h
[modify] https://crrev.com/3da560edaced9383d3dd33f71d6263eadf9444b0/chrome/browser/ui/ash/wallpaper_controller_client.cc
[modify] https://crrev.com/3da560edaced9383d3dd33f71d6263eadf9444b0/chrome/browser/ui/ash/wallpaper_controller_client.h
[add] https://crrev.com/3da560edaced9383d3dd33f71d6263eadf9444b0/chrome/browser/ui/ash/wallpaper_policy_handler.cc
[add] https://crrev.com/3da560edaced9383d3dd33f71d6263eadf9444b0/chrome/browser/ui/ash/wallpaper_policy_handler.h

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/6c610db05e3b2b18ea2066098831cc4c981b0168

commit 6c610db05e3b2b18ea2066098831cc4c981b0168
Author: xdai <xdai@chromium.org>
Date: Wed Jan 03 19:02:12 2018

wallpaper refactoring: hook WallpaperPolicyHandler with WallpaperControllerClient.

It's a follow-up CL of https://chromium-review.googlesource.com/c/chromium/src/+/820854.

Bug:  791654 
Test: Manual, browser_tests
Change-Id: I7e04a6839d1af6897a560a9875280ea7a2c18004
Reviewed-on: https://chromium-review.googlesource.com/833486
Commit-Queue: Xiaoqian Dai <xdai@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Wenzhao (Colin) Zang <wzang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#526755}
[modify] https://crrev.com/6c610db05e3b2b18ea2066098831cc4c981b0168/ash/public/interfaces/wallpaper.mojom
[modify] https://crrev.com/6c610db05e3b2b18ea2066098831cc4c981b0168/ash/wallpaper/wallpaper_controller.cc
[modify] https://crrev.com/6c610db05e3b2b18ea2066098831cc4c981b0168/ash/wallpaper/wallpaper_controller.h
[modify] https://crrev.com/6c610db05e3b2b18ea2066098831cc4c981b0168/chrome/browser/chromeos/arc/arc_util_unittest.cc
[modify] https://crrev.com/6c610db05e3b2b18ea2066098831cc4c981b0168/chrome/browser/chromeos/extensions/wallpaper_private_api_unittest.cc
[modify] https://crrev.com/6c610db05e3b2b18ea2066098831cc4c981b0168/chrome/browser/chromeos/login/users/wallpaper/wallpaper_manager.cc
[modify] https://crrev.com/6c610db05e3b2b18ea2066098831cc4c981b0168/chrome/browser/chromeos/login/users/wallpaper/wallpaper_manager.h
[modify] https://crrev.com/6c610db05e3b2b18ea2066098831cc4c981b0168/chrome/browser/ui/ash/multi_user/multi_user_window_manager_chromeos_unittest.cc
[modify] https://crrev.com/6c610db05e3b2b18ea2066098831cc4c981b0168/chrome/browser/ui/ash/test_wallpaper_controller.cc
[modify] https://crrev.com/6c610db05e3b2b18ea2066098831cc4c981b0168/chrome/browser/ui/ash/test_wallpaper_controller.h
[modify] https://crrev.com/6c610db05e3b2b18ea2066098831cc4c981b0168/chrome/browser/ui/ash/wallpaper_controller_client.cc
[modify] https://crrev.com/6c610db05e3b2b18ea2066098831cc4c981b0168/chrome/browser/ui/ash/wallpaper_controller_client.h
[modify] https://crrev.com/6c610db05e3b2b18ea2066098831cc4c981b0168/chrome/browser/ui/ash/wallpaper_controller_client_unittest.cc
[modify] https://crrev.com/6c610db05e3b2b18ea2066098831cc4c981b0168/chrome/browser/ui/webui/chromeos/login/signin_userlist_unittest.cc