Add support for persistent file access in apps.

chrome/common/extensions/api/_permission_features.json

 // 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.
 
 {
   "activeTab": {
     "channel": "stable",
     "extension_types": ["extension", "packaged_app"],
     "min_manifest_version": 2
   },
@@ skipping 190 matching lines @@
     "location": "component"
   },
   "fileSystem": [{
     "channel": "stable",
     "extension_types": ["platform_app"]
   },{
     "channel": "stable",
     "extension_types": ["extension"],
     "whitelist": [ "2FC374607C2DF285634B67C64A2E356C607091C3" ]
   }],
+  "fileSystem.retainFiles": [{
+    "channel": "trunk",
+    "extension_types": ["platform_app"]
+  }],
   "fileSystem.write": [{
     "channel": "stable",
     "extension_types": ["platform_app"]
   },{
     "channel": "stable",
     "extension_types": ["extension"],
     "whitelist": [ "2FC374607C2DF285634B67C64A2E356C607091C3" ]
   }],
   "fontSettings": {
     "channel": "stable",
@@ skipping 343 matching lines @@
   },
   "webRequestBlocking": {
     "channel": "stable",
     "extension_types": ["extension", "packaged_app"]
   },
   "webview": {
     "channel": "stable",
     "extension_types": ["platform_app"]
   }
 }