Save the wallpaper selected for the user if was not set previsouly (first time).

chrome/browser/chromeos/login/user_manager_impl.cc

Index: chrome/browser/chromeos/login/user_manager_impl.cc
diff --git a/chrome/browser/chromeos/login/user_manager_impl.cc b/chrome/browser/chromeos/login/user_manager_impl.cc
index a5800a51771b635d33e3d4b982e0c2b4a06f7acd..bf365172e4923daf828df612989b3d7d7283aebd 100644
--- a/chrome/browser/chromeos/login/user_manager_impl.cc
+++ b/chrome/browser/chromeos/login/user_manager_impl.cc
@@ -914,8 +914,8 @@ int UserManagerImpl::GetUserWallpaper(const std::string& username) {
   const DictionaryValue* user_wallpapers =
       local_state->GetDictionary(UserManager::kUserWallpapers);
   int index = ash::GetDefaultWallpaperIndex();
-  user_wallpapers->GetIntegerWithoutPathExpansion(username,
-                                                  &index);
+  if (!user_wallpapers->GetIntegerWithoutPathExpansion(username, &index))
+    SaveWallpaperDefaultIndex(username, index);
   return index;
 }