[fsp] Add FileStreamReader for the reading operation.

chrome/common/extensions/api/file_system_provider_internal.idl

 // Copyright 2014 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.
 
 // Internal, used by fileSystemProvider's custom bindings. These functions are
 // called when events' callbacks are invoked.
 [platforms=("chromeos"),
  implemented_in="chrome/browser/chromeos/extensions/file_system_provider/file_system_provider_api.h", nodoc]
 namespace fileSystemProviderInternal {
   interface Functions {
@@ skipping 62 matching lines @@
     static void closeFileRequestedError(
         long fileSystemId,
         long requestId,
         fileSystemProvider.ProviderError error);
 
     // Internal. Success callback of the <code>onReadFileRequested</code>
     // event. Can be called multiple times per request.
     static void readFileRequestedSuccess(
         long fileSystemId,
         long requestId,
-        DOMString data,
+        ArrayBuffer data,
         boolean hasNext);
 
     // Internal. Error callback of the <code>onReadFileRequested</code>
     // event. Must be called when reading a file fails.
     static void readFileRequestedError(
         long fileSystemId,
         long requestId,
         fileSystemProvider.ProviderError error);
   };
 };