Remove a DCHECK that is no longer accurate

When Creating a download item from history db, the item might have already
imported from the in-progress db.
So this DCHECK is no longer correct

BUG=842245

Change-Id: Iff0c7f77f33bd910ba42602a526c914d0e12bcd7
Reviewed-on: https://chromium-review.googlesource.com/1129366Reviewed-by: Xing Liu <xingliu@chromium.org>
Reviewed-by: Min Qin <qinmin@chromium.org>
Commit-Queue: Min Qin <qinmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#573433}

content/browser/download/download_manager_impl.cc

@@ -882,7 +882,6 @@ download::DownloadItem* DownloadManagerImpl::CreateDownloadItem(
       return downloads_[id].get();
     }
   }
-  DCHECK(!base::ContainsKey(downloads_by_guid_, guid));
   download::DownloadItemImpl* item = item_factory_->CreatePersistedItem(
       this, guid, id, current_path, target_path, url_chain, referrer_url,
       site_url, tab_url, tab_refererr_url, mime_type, original_mime_type,