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 URLRequestRedirectInfo;
 
 [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.

[dcheng, 2016/11/17 11:16:02]

Empty interfaces are a bit unusual. My main feedback is it would be useful for
the interface to explicitly describe why it's empty (it's an opaque handle),
which way the handle is passed (browser -> child), and explicitly what happens
when we close the pipe (the temporary file is deleted... I think?). Strawman
suggestion:

// Opaque handle passed from the browser process to a child
// process to manage the lifetime of temporary files used for
// download_to_file resource loading. When the message pipe
// for this interface is closed, the browser process will
// clean up the corresponding temporary file.

[tzik, 2016/11/22 13:54:45]

Done.

+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

[dcheng, 2016/11/17 11:16:02]

Nit: on => in

[tzik, 2016/11/22 13:54:45]

Done.

+  // the 'download_to_file' case.
+  OnReceiveResponse(URLResponseHead head, DownloadedTempFile? downloaded_file);
 
   // Called when the request has been redirected. The receiver is expected to
   // call FollowRedirect or cancel the request.
   OnReceiveRedirect(URLRequestRedirectInfo redirect_info, URLResponseHead head);
 
   // 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);
 };