Enable fileSystem.retainFiles on dev.

BUG=224684

Review URL: https://chromiumcodereview.appspot.com/18029017

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@209311 0039d316-1c4b-4281-b951-d872f2087c98

chrome/common/extensions/api/_permission_features.json

@@ -245,7 +245,7 @@
     "whitelist": [ "2FC374607C2DF285634B67C64A2E356C607091C3" ]
   }],
   "fileSystem.retainFiles": [{
-    "channel": "trunk",
+    "channel": "dev",
     "extension_types": ["platform_app"]
   }],
   "fileSystem.write": [{