Docserver: Add extensionTypes to _api_features.json

Review URL: https://codereview.chromium.org/531973003

Cr-Commit-Position: refs/heads/master@{#293051}

chrome/common/extensions/api/_api_features.json

@@ -387,6 +387,13 @@
   }],
   // This is not a real API, only here for documentation purposes.
   // See http://crbug.com/275944 for background.
+  "extensionTypes": {
+    "internal": true,
+    "channel": "stable",
+    "contexts": ["blessed_extension"]
+  },
+  // This is not a real API, only here for documentation purposes.
+  // See http://crbug.com/275944 for background.
   "extensionsManifestTypes": {
     "internal": true,
     "channel": "stable",