Check for null web_contents in OfflinePageUtils::ScheduleDownload

Bug: 942426
Change-Id: I24f43ac82827633b6390e012b9a583b364c9167b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1532673Reviewed-by: Peter Williamson <petewil@chromium.org>
Commit-Queue: Jian Li <jianli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#642672}

chrome/browser/offline_pages/offline_page_utils.cc

@@ -328,7 +328,8 @@ void OfflinePageUtils::ScheduleDownload(content::WebContents* web_contents,
                                         const GURL& url,
                                         DownloadUIActionFlags ui_action,
                                         const std::string& request_origin) {
-  DCHECK(web_contents);
+  if (!web_contents)
+    return;
 
   // Ensure that the storage permission is granted since the archive file is
   // going to be placed in the public directory.