[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}
void WallpaperManagerBase::NotifyAnimationFinished() {
  for (auto& observer : observers_)
    observer.OnWallpaperAnimationFinished(last_selected_user_);
}

void WallpaperManagerBase::NotifyAnimationFinished() {
  for (auto& observer : observers_)
    observer.OnWallpaperAnimationFinished(last_selected_user_);
}
