Implement externally_connectable! Web pages can now communicate directly with

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.
 
+// See c/c/e/features/* to understand this file, in particular feature.h,
+// simple_feature.h, and base_feature_provider.h.
+
 {
   "app": {
     "channel": "stable",
     "extension_types": ["packaged_app", "hosted_app", "platform_app"]
   },
   // The default platform app CSP can only be overridden by whitelisted apps.
   // This is a separate key from the top-level content_security_policy one since
   // we can't combine type restrictions with whitelisted ID restrictions. If
   // there is a need for additional whitelisted entries, the feature system
   // should instead be extended to support OR-ing of restrictions.
@@ skipping 321 matching lines @@
     "channel": "stable",
     "extension_types": "all"
   },
   "web_accessible_resources": {
     "channel": "stable",
     "extension_types": [
       "extension", "packaged_app", "hosted_app"
     ]
   }
 }