Fix DCHECK crash in CheckClientDownloadRequest::OnDownloadDestroyed

For some rare cases, service_ in CheckClientDownloadRequest is already
set to null when OnDownloadDestroyed is called, which as a result will
make item_ still dangle there, and causes crash at the DCHECK(item_==NULL).

Bug: 806785
Change-Id: I384f66ca94d6695d870a632d52b9be26a0f9a3a3
Reviewed-on: https://chromium-review.googlesource.com/891808Reviewed-by: Jialiu Lin <jialiul@chromium.org>
Reviewed-by: Luke Z <lpz@chromium.org>
Commit-Queue: Jialiu Lin <jialiul@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532973}

chrome/browser/safe_browsing/download_protection/check_client_download_request.cc

@@ -1031,6 +1031,7 @@ void CheckClientDownloadRequest::FinishRequest(
     // so we may be deleted now.
   } else {
     callback_.Run(DownloadCheckResult::UNKNOWN);
+    item_ = NULL;
   }
 }