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}
  TestingDomainReliabilityServiceFactoryUserData(
      content::BrowserContext* context,
      MockDomainReliabilityService* service)
      : context(context),
        service(service),
        attached(false) {}

  TestingDomainReliabilityServiceFactoryUserData(
      content::BrowserContext* context,
      MockDomainReliabilityService* service)
      : context(context),
        service(service),
        attached(false) {}
