Initialize download service instance.

Create the download service instance to build the correct dependency
tree for keyed services.


Bug: 
Change-Id: Ia29bf896acca7c387698cd9b35269e3c6caf2591
Reviewed-on: https://chromium-review.googlesource.com/544760Reviewed-by: Elliot Glaysher <erg@chromium.org>
Reviewed-by: David Trainor <dtrainor@chromium.org>
Commit-Queue: Xing Liu <xingliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#485403}

chrome/browser/profiles/chrome_browser_main_extra_parts_profiles.cc

@@ -22,6 +22,7 @@
 #include "chrome/browser/dom_distiller/dom_distiller_service_factory.h"
 #include "chrome/browser/domain_reliability/service_factory.h"
 #include "chrome/browser/download/download_core_service_factory.h"
+#include "chrome/browser/download/download_service_factory.h"
 #include "chrome/browser/engagement/site_engagement_service.h"
 #include "chrome/browser/engagement/site_engagement_service_factory.h"
 #include "chrome/browser/favicon/favicon_service_factory.h"
@@ -224,6 +225,7 @@ EnsureBrowserContextKeyedServiceFactoriesBuilt() {
   dom_distiller::DomDistillerServiceFactory::GetInstance();
   domain_reliability::DomainReliabilityServiceFactory::GetInstance();
   DownloadCoreServiceFactory::GetInstance();
+  DownloadServiceFactory::GetInstance();
 #if BUILDFLAG(ENABLE_EXTENSIONS)
   EasyUnlockServiceFactory::GetInstance();
   EnhancedBookmarkKeyServiceFactory::GetInstance();