Fix a download extension crash for manifest V3

Manifest v3 use service workers, so there is no RenderFrameHost.

BUG=1168219

Change-Id: Idb777abe6b2be1fff9175271d30dfba7b682eceb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2644100Reviewed-by: Xing Liu <xingliu@chromium.org>
Commit-Queue: Min Qin <qinmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#845867}

chrome/browser/extensions/api/downloads/downloads_api.cc

@@ -1036,7 +1036,8 @@ ExtensionFunction::ResponseAction DownloadsDownloadFunction::Run() {
   std::unique_ptr<download::DownloadUrlParameters> download_params(
       new download::DownloadUrlParameters(
           download_url, source_process_id(),
-          render_frame_host()->GetRoutingID(), traffic_annotation));
+          render_frame_host() ? render_frame_host()->GetRoutingID() : -1,
+          traffic_annotation));
 
   base::FilePath creator_suggested_filename;
   if (options.filename.get()) {