This is the IPC and bits of the browser backend for FileWriter. The rest of...

chrome/browser/file_system/browser_file_system_callback_dispatcher.cc

 // Copyright (c) 2010 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.
 
 #include "chrome/browser/file_system/browser_file_system_callback_dispatcher.h"
 
 #include "chrome/browser/file_system/file_system_dispatcher_host.h"
 
 BrowserFileSystemCallbackDispatcher::BrowserFileSystemCallbackDispatcher(
     FileSystemDispatcherHost* dispatcher_host, int request_id)
@@ skipping 25 matching lines @@
     const string16&, const FilePath&) {
   NOTREACHED();
 }
 
 void BrowserFileSystemCallbackDispatcher::DidFail(
     base::PlatformFileError error_code) {
   dispatcher_host_->Send(new ViewMsg_FileSystem_DidFail(
       request_id_, error_code));
   dispatcher_host_->RemoveCompletedOperation(request_id_);
 }
+
+void BrowserFileSystemCallbackDispatcher::DidWrite(
+    int64 bytes,
+    bool complete) {
+  dispatcher_host_->Send(new ViewMsg_FileSystem_DidWrite(
+      request_id_, bytes, complete));
+  if (complete)
+    dispatcher_host_->RemoveCompletedOperation(request_id_);
+}