[reland] Do not set default wallpaper unless it should do so.

TBR=bshe@chromium.org, alemate@chromium.org

Bug: 751382
Change-Id: Id0793dfe467f737526a95b1e66ed01fbb8860bda
Reviewed-on: https://chromium-review.googlesource.com/619754
Commit-Queue: Xiaoqian Dai <xdai@chromium.org>
Reviewed-by: Alexander Alekseev <alemate@chromium.org>
Reviewed-by: Biao She <bshe@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#498325}
Reviewed-on: https://chromium-review.googlesource.com/646430
Cr-Commit-Position: refs/heads/master@{#498982}
base::FilePath WallpaperManagerBase::GetCustomWallpaperPath(
    const char* sub_dir,
    const wallpaper::WallpaperFilesId& wallpaper_files_id,
    const std::string& file) {
  base::FilePath custom_wallpaper_path = GetCustomWallpaperDir(sub_dir);
  return custom_wallpaper_path.Append(wallpaper_files_id.id()).Append(file);
}

base::FilePath WallpaperManagerBase::GetCustomWallpaperPath(
    const char* sub_dir,
    const wallpaper::WallpaperFilesId& wallpaper_files_id,
    const std::string& file) {
  base::FilePath custom_wallpaper_path = GetCustomWallpaperDir(sub_dir);
  return custom_wallpaper_path.Append(wallpaper_files_id.id()).Append(file);
}
