Use the externally_connectable schema from manifest_types.json to generate its

chrome/common/extensions/api/_api_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.
 
 {
   "activityLogPrivate": {
     "dependencies": ["permission:activityLogPrivate"],
@@ skipping 309 matching lines @@
   "management.getPermissionWarningsByManifest": {
     "dependencies": [],
     "channel": "stable",
     "extension_types": ["extension", "packaged_app", "platform_app"]
   },
   "management.uninstallSelf": {
     "dependencies": [],
     "channel": "stable",
     "extension_types": ["extension", "packaged_app", "platform_app"]
   },
+  // This is not a real API, only here for documentation purposes.
+  // See http://crbug.com/275944 for background.
+  "manifestTypes": {
+    "internal": true,
+    "channel": "stable",
+    "contexts": "all"
+  },
   "mediaGalleries": {
     "dependencies": ["permission:mediaGalleries"],
     "contexts": ["blessed_extension"]
   },
   "mediaGalleriesPrivate": {
     "dependencies": ["permission:mediaGalleriesPrivate"],
     "contexts": ["blessed_extension"]
   },
   "mediaPlayerPrivate": {
     "platform": "chromeos",
@@ skipping 248 matching lines @@
   },
   "webview": {
     "dependencies": ["permission:webview"],
     "contexts": ["blessed_extension"]
   },
   "windows": {
     "dependencies": ["api:tabs"],
     "contexts": ["blessed_extension"]
   }
 }