Fix a use-after-free issue

ChromeDownloadManagerDelegate is owned by DownloadCoreServiceImpl.
So it may outlive DownloadManagerImpl. This CL fixes the use-after-free
issue by checking whether DownloadManagerImpl is still alive.

BUG=1142069

Change-Id: I6d7e39c3911ec6a27be3d8a99463929cc2a45527
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2504591Reviewed-by: Xing Liu <xingliu@chromium.org>
Reviewed-by: Daniel Rubery <drubery@chromium.org>
Commit-Queue: Min Qin <qinmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#821544}

chrome/browser/download/chrome_download_manager_delegate.cc

@@ -1241,6 +1241,8 @@ void ChromeDownloadManagerDelegate::GetFileMimeType(
 void ChromeDownloadManagerDelegate::CheckClientDownloadDone(
     uint32_t download_id,
     safe_browsing::DownloadCheckResult result) {
+  if (!download_manager_)
+    return;
   DownloadItem* item = download_manager_->GetDownload(download_id);
   if (!item || (item->GetState() != DownloadItem::IN_PROGRESS &&
                 item->GetDangerType() !=