Downloads: replace BrowserThread::FILE with task scheduler.

content/browser/download/save_file_manager.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.
 //
 // Objects that handle file operations for saving files, on the file thread.
 //
 // The SaveFileManager owns a set of SaveFile objects, each of which connects
 // with a SaveItem object which belongs to one SavePackage and runs on the file
 // thread for saving data in order to avoid disk activity on either network IO
 // thread or the UI thread. It coordinates the notifications from the network
 // and UI.
 //
 // The SaveFileManager itself is a singleton object owned by the
 // ResourceDispatcherHostImpl.
 //
 // The data sent to SaveFileManager have 2 sources, one is from
 // ResourceDispatcherHostImpl, run in network IO thread, the all sub-resources
 // and save-only-HTML pages will be got from network IO. The second is from
 // render process, those html pages which are serialized from DOM will be
 // composed in render process and encoded to its original encoding, then sent
 // to UI loop in browser process, then UI loop will dispatch the data to
 // SaveFileManager on the file thread. SaveFileManager will directly
 // call SaveFile's method to persist data.
 //
-// A typical saving job operation involves multiple threads:
+// A typical saving job operation involves multiple threads and sequences:
 //
 // Updating an in progress save file
 // io_thread
 //      |----> data from net   ---->|
 //                                  |
 //                                  |
 //      |----> data from    ---->|  |
 //      |      render process    |  |
 // ui_thread                     |  |
-//                      file_thread (writes to disk)
-//                              |----> stats ---->|
+//                   download_task_runner (writes to disk)
+//                               |----> stats ---->|
 //                                              ui_thread (feedback for user)
 //
 //
 // Cancel operations perform the inverse order when triggered by a user action:
 // ui_thread (user click)
 //    |----> cancel command ---->|
-//    |           |      file_thread (close file)
+//    |           |      download_task_runner (close file)
 //    |           |---------------------> cancel command ---->|
 //    |                                               io_thread (stops net IO
 // ui_thread (user close contents)                               for saving)
 //    |----> cancel command ---->|
 //                            Render process(stop serializing DOM and sending
 //                                           data)
 //
 //
 // The SaveFileManager tracks saving requests, mapping from a save item id to
 // the SavePackage for the contents where the saving job was initiated. In the
@@ skipping 162 matching lines @@
   // Tracks which SavePackage to send data to, called only on UI thread.
   // SavePackageMap maps save item ids to their SavePackage.
   std::unordered_map<SaveItemId, SavePackage*, SaveItemId::Hasher> packages_;
 
   DISALLOW_COPY_AND_ASSIGN(SaveFileManager);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_DOWNLOAD_SAVE_FILE_MANAGER_H_