Fix issues and feature polishing for parallel download.

content/browser/download/download_create_info.h

 // Copyright (c) 2012 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.
 
 #ifndef CONTENT_BROWSER_DOWNLOAD_DOWNLOAD_CREATE_INFO_H_
 #define CONTENT_BROWSER_DOWNLOAD_DOWNLOAD_CREATE_INFO_H_
 
 #include <stdint.h>
 
 #include <string>
@@ skipping 46 matching lines @@
   // The referrer URL of the tab that started us.
   GURL tab_referrer_url;
 
   // The time when the download started.
   base::Time start_time;
 
   // The size of the response body. If content-length response header is not
   // presented or can't be parse, set to 0.
   int64_t total_bytes;
 
-  // The starting position of the request to save to the target file.
+  // The starting position of the initial request.
   // This value matches the offset in DownloadSaveInfo.
-  // TODO(xingliu): Remove this after we refactor DownloadJob to own
-  // DownloadFile.
+  // TODO(xingliu): Refactor to remove |offset| and |length|.
   int64_t offset;
 
+  // The length of the initial request.
+  // This value matches the length in DownloadSaveInfo.
+  int64_t length;

[qinmin, 2017/03/31 17:56:09]

the initial request is always half open, right? why we need this?

[xingliu, 2017/03/31 21:46:10]

Done, addressed in
https://codereview.chromium.org/2789623005/

Also removed the logic to send requests for slices before initial request. Just
use a DCHECK now.

+
   // True if the download was initiated by user action.
   bool has_user_gesture;
 
   base::Optional<ui::PageTransition> transition_type;
 
   // The remote IP address where the download was fetched from.  Copied from
   // UrlRequest::GetSocketAddress().
   std::string remote_address;
 
   // If the download is initially created in an interrupted state (because the
@@ skipping 41 matching lines @@
   // The HTTP connection type.
   net::HttpResponseInfo::ConnectionInfo connection_info;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(DownloadCreateInfo);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_DOWNLOAD_DOWNLOAD_CREATE_INFO_H_