Issue 857563  has been merged into this issue.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/e9c0963c5214fbcbf4f5adb3469413283191d6a4

commit e9c0963c5214fbcbf4f5adb3469413283191d6a4
Author: Wenzhao Zang <wzang@chromium.org>
Date: Fri Jun 29 18:12:07 2018

cros: ash::WallpaperDecoder should be able to decode png images

This issue is not very easy to fix since:
- The png decoder is not reliable according to [1]
- The default codec is considered "unsafe", confirmed by multiple
  sources (e.g. [2], [3])

So I think we should continue to use the jpeg decoder on login screen
(it's always working because we re-encode all the wallpaper images in
jpeg [4]) and make an "exception" to use the default codec only when
it's safe. This will solve the issue because online wallpapers used to
be decoded by the code point at [3] before the refactoring, which uses
default codec.

[1]
https://cs.chromium.org/chromium/src/ui/gfx/codec/png_codec.h?dr=Ss&g=0&l=24
[2]
https://cs.chromium.org/chromium/src/chrome/browser/chromeos/login/users/avatar/user_image_manager_impl.cc?type=cs&q=DEFAULT_CODEC&sq=package:chromium&g=0&l=386
[3]
https://cs.chromium.org/chromium/src/chrome/browser/chromeos/extensions/wallpaper_function_base.cc?q=wallpaper_function_base&sq=package:chromium&dr=C&l=81
[4]
https://cs.chromium.org/chromium/src/ash/wallpaper/wallpaper_controller.cc?type=cs&g=0&l=207

Bug:  857240 
Change-Id: I1bb0f1351a72acec233539e08c43221c8952d032
Reviewed-on: https://chromium-review.googlesource.com/1117764
Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Commit-Queue: Wenzhao (Colin) Zang <wzang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#571546}
[modify] https://crrev.com/e9c0963c5214fbcbf4f5adb3469413283191d6a4/ash/wallpaper/wallpaper_controller.cc
[modify] https://crrev.com/e9c0963c5214fbcbf4f5adb3469413283191d6a4/ash/wallpaper/wallpaper_utils/wallpaper_decoder.cc
[modify] https://crrev.com/e9c0963c5214fbcbf4f5adb3469413283191d6a4/ash/wallpaper/wallpaper_utils/wallpaper_decoder.h