Make platform apps get isolated storage by default.

chrome/common/extensions/api/_manifest_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.
 
 {
   "app": {
     "channel": "stable",
     "extension_types": ["packaged_app", "hosted_app", "platform_app"]
   },
   "app.launch": {
     "channel": "stable",
     "extension_types": ["packaged_app", "hosted_app"]
   },
+  "app.isolation": {
+    "channel": "stable",
+    // Platform apps always have isolated storage, thus they cannot specify it
+    // via the manifest.
+    "extension_types": ["packaged_app", "hosted_app"]
+  },
   "background": {
     "channel": "stable",
     "extension_types": [
       "extension", "packaged_app", "hosted_app", "platform_app"
     ]
   },
   "background.persistent": {
     "channel": "dev",
     "extension_types": [
       "extension", "packaged_app", "platform_app"
@@ skipping 177 matching lines @@
     "channel": "stable",
     "extension_types": "all"
   },
   "web_accessible_resources": {
     "channel": "stable",
     "extension_types": [
       "extension", "packaged_app", "hosted_app"
     ]
   }
 }