Move all wallpaper file loading and decoding from DesktopBackgroundController to WallpaperManager.

ash/test/test_user_wallpaper_delegate.cc

Index: ash/test/test_user_wallpaper_delegate.cc
diff --git a/ash/test/test_user_wallpaper_delegate.cc b/ash/test/test_user_wallpaper_delegate.cc
index 24ab287e5636d4830182646399ffcf690c491229..f1d0b9f7f0b37c303e82e43015354df3f7de6cc9 100644
--- a/ash/test/test_user_wallpaper_delegate.cc
+++ b/ash/test/test_user_wallpaper_delegate.cc
@@ -18,7 +18,7 @@ TestUserWallpaperDelegate::~TestUserWallpaperDelegate() {}
 void TestUserWallpaperDelegate::UpdateWallpaper(bool clear_cache) {
   DefaultUserWallpaperDelegate::UpdateWallpaper(clear_cache);
   if (!custom_wallpaper_.isNull()) {
-    Shell::GetInstance()->desktop_background_controller()->SetCustomWallpaper(
+    Shell::GetInstance()->desktop_background_controller()->SetWallpaper(
         custom_wallpaper_, WALLPAPER_LAYOUT_STRETCH);
   }
   update_wallpaper_count_++;