Enable fileSystem.retainFiles on dev.

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.
 
 // See c/c/e/features/* to understand this file, in particular feature.h,
 // simple_feature.h, and base_feature_provider.h.
 
 {
   "activeTab": {
     "channel": "stable",
@@ skipping 227 matching lines @@
   },
   "fileSystem": [{
     "channel": "stable",
     "extension_types": ["platform_app"]
   },{
     "channel": "stable",
     "extension_types": ["extension"],
     "whitelist": [ "2FC374607C2DF285634B67C64A2E356C607091C3" ]
   }],
   "fileSystem.retainFiles": [{
-    "channel": "trunk",
+    "channel": "dev",
     "extension_types": ["platform_app"]
   }],
   "fileSystem.write": [{
     "channel": "stable",
     "extension_types": ["platform_app"]
   },{
     "channel": "stable",
     "extension_types": ["extension"],
     "whitelist": [ "2FC374607C2DF285634B67C64A2E356C607091C3" ]
   }],
@@ skipping 381 matching lines @@
   },
   "webRequestBlocking": {
     "channel": "stable",
     "extension_types": ["extension", "packaged_app"]
   },
   "webview": {
     "channel": "stable",
     "extension_types": ["platform_app"]
   }
 }