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.runtime.getBackgroundPage(
+        chrome.test.callbackPass(function(backgroundPage) {
+      backgroundPage.callback = chrome.test.callbackPass(

[koz (OOO until 15th September), 2013/05/24 00:13:16]

This callback should go down to where chrome.app.window.create() is and add a
comment to explain that we are stashing it on the background page so it can be
called from the other window. Alternatively, have you considered using the
callback from chrome.app.window.create() to stash this callback on the other
window itself?

[Sam McNally, 2013/05/24 00:46:03]

Done.

+          function(other_window, id, entry) {
+        other_window.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 */);
+        }));
+      });
+      chrome.app.window.create('test_other_window.html');
+    }));
+  }
+]);