Fix ManifestUpgradeDetector logic when manifest has an icon URL but the icon URL does not point to an image

In particular, we need to make ManifestUpgradeDetector take into account:

void AddToHomescreenDataFetcher::OnManifestIconFetched(const GURL& icon_url,
                                                       const SkBitmap& icon) {
  if (icon.drawsNothing()) {
    FetchFavicon();
    return;
  }
  shortcut_info_.icon_url = icon_url;
  NotifyObserver(icon);
}