Implement declarativeContent API.

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": "dev",
     "extension_types": ["extension", "packaged_app"],
     "min_manifest_version": 2
   },
@@ skipping 100 matching lines @@
     "extension_types": ["extension", "packaged_app"]
   },
   "debugger": {
     "channel": "stable",
     "extension_types": ["extension", "packaged_app"]
   },
   "devtools": {
     "channel": "stable",
     "extension_types": ["extension", "packaged_app"]
   },
+  "declarativeContent": {
+    "channel": "trunk",
+    "extension_types": ["extension"]
+  },
   "declarativeWebRequest": {
     "channel": "beta",
     "extension_types": ["extension", "packaged_app"]
   },
   "downloads": {
     "channel": "dev",
     "extension_types": [
       "extension", "packaged_app"
     ]
   },
@@ skipping 282 matching lines @@
   },
   "webRequestBlocking": {
     "channel": "stable",
     "extension_types": ["extension", "packaged_app"]
   },
   "webview": {
     "channel": "stable",
     "extension_types": ["platform_app"]
   }
 }