Add support for persistent file access in apps.

chrome/test/data/extensions/platform_apps/launch_file/test.js

 // 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.
 
 chrome.app.runtime.onLaunched.addListener(function (launchData) {
   // Test that the id and items fields in FileEntry can be read.
   chrome.test.runTests([
     function testFileHandler() {
       chrome.test.assertFalse(!launchData, "No launchData");
       chrome.test.assertEq(launchData.id, "text",
           "launchData.id incorrect");
       chrome.test.assertEq(launchData.items.length, 1);
       chrome.test.assertTrue(
-          chrome.fileSystem.getEntryId(launchData.items[0].entry) != null);
+          chrome.fileSystem.retainEntry(launchData.items[0].entry) != null);
 
       launchData.items[0].entry.file(function(file) {
         var reader = new FileReader();
         reader.onloadend = function(e) {
           chrome.test.assertEq(
               reader.result.indexOf("This is a test. Word."), 0);
           chrome.test.succeed();
         };
         reader.onerror = function(e) {
           chrome.test.fail("Error reading file contents.");
         };
         reader.readAsText(file);
       });
     }
   ]);
 });