[fsp] Introduce BufferingFileStreamReader to read files in bigger chunks.

chrome/test/data/extensions/api_test/file_system_provider/big_file/test.js

 // 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.
 
 'use strict';
 
 /**
  * @type {DOMFileSystem}
  */
 var fileSystem = null;
@@ skipping 127 matching lines @@
  */
 function onReadFileRequested(options, onSuccess, onError) {
   var filePath = openedFiles[options.openRequestId];
   if (options.fileSystemId != test_util.FILE_SYSTEM_ID || !filePath) {
     onError('INVALID_OPERATION');  // enum ProviderError.
     return;
   }
 
   if (filePath == '/' + TESTING_6GB_FILE.name) {
     if (options.offset < TESTING_TEXT_OFFSET ||
-        options.offset + options.length >
-            TESTING_TEXT_OFFSET + TESTING_TEXT.length) {
+        options.offset >= TESTING_TEXT_OFFSET + TESTING_TEXT.length) {
       console.error('Reading from a wrong location in the file!');
-      onError('INVALID_FAILED');  // enum ProviderError.
+      onError('FAILED');  // enum ProviderError.
       return;
     }
 
     var buffer = TESTING_TEXT.substr(
         options.offset - TESTING_TEXT_OFFSET, options.length);
     var reader = new FileReader();
     reader.onload = function(e) {
       onSuccess(e.target.result, false /* hasMore */);
     };
     reader.readAsArrayBuffer(new Blob([buffer]));
@@ skipping 59 matching lines @@
           },
           function(error) {
             chrome.test.fail(error.name);
           });
     }
   ]);
 }
 
 // Setup and run all of the test cases.
 setUp(runTests);