Add support for persistent file access in apps.

chrome/test/data/extensions/api_test/file_system/retain_entry/test.js

+// 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.
+
+chrome.test.runTests([
+  function retainEntryWorks() {
+    chrome.app.window.create('test_other_window.html', chrome.test.callbackPass(
+        function(otherWindow) {
+      otherWindow.contentWindow.callback = chrome.test.callbackPass(
+          function(id, entry) {
+        otherWindow.close();
+        chrome.fileSystem.isRestorable(id, chrome.test.callbackPass(
+            function(isRestorable) {
+          chrome.test.assertTrue(isRestorable);
+        }));
+        chrome.test.assertEq(chrome.fileSystem.retainEntry(entry), id);
+        chrome.fileSystem.restoreEntry(id, chrome.test.callbackPass(
+            function(restoredEntry) {
+          chrome.test.assertEq(restoredEntry, entry);
+          chrome.test.assertEq(
+              chrome.fileSystem.retainEntry(restoredEntry), id);
+          checkEntry(restoredEntry, 'writable.txt', false /* isNew */,
+            false /*shouldBeWritable */);
+        }));
+      });
+    }));
+  }
+]);