Allow features to refer to subkeys in _manifest_features.json.

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"]
   },
   "background": {
     "channel": "stable",
     "extension_types": [
       "extension", "packaged_app", "hosted_app", "platform_app"
     ]
   },
+  "background.persistent": {
+    "channel": "dev",
+    "extension_types": [
+      "extension", "packaged_app", "platform_app"
+    ],
+    "min_manifest_version": 2
+  },
   "background_page": {
     "channel": "stable",
     "extension_types": [
       "extension", "packaged_app", "hosted_app", "platform_app"
     ],
     "max_manifest_version": 1
   },
   "browser_action": {
     "channel": "stable",
     "extension_types": ["extension"]
@@ skipping 164 matching lines @@
     "channel": "stable",
     "extension_types": "all"
   },
   "web_accessible_resources": {
     "channel": "stable",
     "extension_types": [
       "extension", "packaged_app", "hosted_app", "platform_app"
     ]
   }
 }