[fsp] Add FileStreamReader for the reading operation.

chrome/common/extensions/api/file_system_provider.idl

 // Copyright 2013 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.
 
 // Use the <code>chrome.fileSystemProvider</code> API to create file systems,
 // that can be accessible from the file manager on Chrome OS.
 [platforms=("chromeos"),
  implemented_in="chrome/browser/chromeos/extensions/file_system_provider/file_system_provider_api.h"]
 namespace fileSystemProvider {
   // Error codes used by providing extensions in response to requests. For
@@ skipping 67 matching lines @@
   // Success callback for the <code>onReadDirectoryRequested</code> event. If
   // more entries will be returned, then <code>hasNext</code> must be true, and
   // it has to be called again with additional entries. If no more entries are
   // available, then <code>hasNext</code> must be set to false.
   callback EntriesCallback = void(ResourceEntry[] entries, bool hasNext);
 
   // Success callback for the <code>onReadFileRequested</code> event. If more
   // data will be returned, then <code>hasNext</code> must be true, and it
   // has to be called again with additional entries. If no more data is
   // available, then <code>hasNext</code> must be set to false.
-  callback FileDataCallback = void(DOMString data, bool hasNext);
+  callback FileDataCallback = void(ArrayBuffer data, bool hasNext);
 
   interface Functions {
     // Mounts a file system with the given <code>displayName</code>.
     // <code>displayName</code> will be shown in the left panel of
     // Files.app. <code>displayName</code> can contain any characters
     // including '/', but cannot be an empty string. <code>displayName</code>
     // should be descriptive but doesn't have to be unique. Duplicate display
     // names are uniquified by adding suffix like "(1)" in the Files.app UI.
     static void mount(DOMString displayName,
                       MountCallback successCallback,
@@ skipping 67 matching lines @@
     [maxListeners=1] static void onReadFileRequested(
         long fileSystemId,
         long openRequestId,
         double offset,
         double length,
         FileDataCallback successCallback,
         ProviderErrorCallback errorCallback);
   };
 };