Add content pack information to Extension.

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"]
   },
   // The default platform app CSP can only be overridden by whitelisted apps.
@@ skipping 58 matching lines @@
   },
   "chrome_url_overrides": {
     "channel": "stable",
     "extension_types": ["extension", "packaged_app"]
   },
   "commands": {
     "channel": "stable",
     "extension_types": ["extension", "packaged_app", "platform_app"],
     "min_manifest_version": 2
   },
+  "content_pack": {
+    "channel": "dev",
+    "extension_types": ["extension"]
+  },
   "content_security_policy": {
     "channel": "stable",
     // Platform apps have a restricted content security policy that cannot be
     // overriden (except for a whitelist of exceptions, see the
     // app.content_security_policy whitelist).
     "extension_types": ["extension", "packaged_app"]
   },
   "content_scripts": {
     "channel": "stable",
     "extension_types": ["extension", "packaged_app"]
@@ skipping 173 matching lines @@
     "channel": "stable",
     "extension_types": "all"
   },
   "web_accessible_resources": {
     "channel": "stable",
     "extension_types": [
       "extension", "packaged_app", "hosted_app"
     ]
   }
 }