Revert "cros: Change wallpaper decoding format to default"

This reverts commit 43112301.

Reason for revert: There's occasional decoding failure on login screen.
Will have to revert this CL first and find a better fix for the original bug.

Original change's description:
> cros: Change wallpaper decoding format to default
> 
> This issue did not show up earlier because the old wallpaper picker
> only contained jpeg images.
> 
> Also changes |shrink_to_fit| to true.
> 
> Bug: 852844
> Change-Id: Ia26a091ee38a6de43454eda0243ff24386e77da8
> Reviewed-on: https://chromium-review.googlesource.com/1101400
> Reviewed-by: Weidong Guo <weidongg@chromium.org>
> Commit-Queue: Wenzhao (Colin) Zang <wzang@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#567435}

TBR=wzang@chromium.org,weidongg@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 852844
Change-Id: Iede6a0c39b14ea7a69f980ea6d47d39628ba31ac
Reviewed-on: https://chromium-review.googlesource.com/1117398Reviewed-by: Wenzhao (Colin) Zang <wzang@chromium.org>
Commit-Queue: Wenzhao (Colin) Zang <wzang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#570866}

ash/wallpaper/wallpaper_utils/wallpaper_decoder.cc

@@ -40,8 +40,8 @@ void DecodeWallpaper(const std::string& image_data,
   std::vector<uint8_t> image_bytes(image_data.begin(), image_data.end());
   data_decoder::DecodeImage(
       Shell::Get()->shell_delegate()->GetShellConnector(),
-      std::move(image_bytes), data_decoder::mojom::ImageCodec::DEFAULT,
-      true /* shrink_to_fit */, kMaxImageSizeInBytes,
+      std::move(image_bytes), data_decoder::mojom::ImageCodec::ROBUST_JPEG,
+      false /* shrink_to_fit */, kMaxImageSizeInBytes,
       gfx::Size() /* desired_image_frame_size */,
       base::BindOnce(&ConvertToImageSkia, std::move(callback)));
 }