Use the externally_connectable schema from manifest_types.json to generate its

chrome/common/extensions/api/_api_features.json

Index: chrome/common/extensions/api/_api_features.json
diff --git a/chrome/common/extensions/api/_api_features.json b/chrome/common/extensions/api/_api_features.json
index 548703443533712014bb2cc46a472a348fea0cbb..4ddc01bf13c79b6714a6f6f5c33bad1aded90446 100644
--- a/chrome/common/extensions/api/_api_features.json
+++ b/chrome/common/extensions/api/_api_features.json
@@ -327,6 +327,13 @@
     "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"]