Don't downcast DownloadManagerDelegate to ChromeDownloadManagerDelegate.

DownloadManager has public SetDelegate method and tests and or other subsystems
can install their own implementations of the delegate.

Bug: 805905
Change-Id: Iecf1e0aceada0e1048bed1e2d2ceb29ca64295b8
TBR: tests updated to follow the API change.
Reviewed-on: https://chromium-review.googlesource.com/894702
Reviewed-by: David Vallet <dvallet@chromium.org>
Reviewed-by: Min Qin <qinmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#533515}
  void AddHistory(const GURL& url, base::Time time) {
    history_service_->AddPage(url, time, nullptr, 0, GURL(),
                              history::RedirectList(), ui::PAGE_TRANSITION_LINK,
                              history::SOURCE_BROWSED, false);
  }

  void AddHistory(const GURL& url, base::Time time) {
    history_service_->AddPage(url, time, nullptr, 0, GURL(),
                              history::RedirectList(), ui::PAGE_TRANSITION_LINK,
                              history::SOURCE_BROWSED, false);
  }
