Fix and refactor downloaded file handling in the loading stack

content/common/url_loader.mojom

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 module content.mojom;
 
 [Native]
 struct URLRequest;
 
 [Native]
 struct URLResponseHead;
 
 [Native]
 struct URLLoaderStatus;
 
 // Destroying a URLLoader will cancel the associated request.
 interface URLLoader {
   // If the associated request has |auto_follow_redirects| set to false,
   // then upon receiving an URLResponse with a non-NULL |redirect_url| field,
   // |FollowRedirect| may be called to load the URL indicated by the redirect.
   FollowRedirect();
 };
 
+// Represents a temporary file for a download_to_file case of resource loading.
+// The downloaded file keeps alive until the instance is destroyed.
+interface DownloadedTempFile {
+};
+
 interface URLLoaderClient {
-  // Called when the response head is received.
-  OnReceiveResponse(URLResponseHead head);
+  // Called when the response head is received. |downloaded_file| is non-null on
+  // the 'download_to_file' case.
+  OnReceiveResponse(URLResponseHead head, DownloadedTempFile? downloaded_file);
 
   // Called when some data from a resource request has been downloaded to the
   // file. This is only called in the 'download_to_file' case and replaces
   // OnStartLoadingResponseBody in the call sequence in that case.
   OnDataDownloaded(int64 data_length, int64 encoded_length);
 
   // Called when the loader starts loading response body.
   OnStartLoadingResponseBody(handle<data_pipe_consumer> body);
 
   // Called when the loading completes. No notification will be dispatched for
   // this client after this message arrives.
   OnComplete(URLLoaderStatus completion_status);
 };