When generating externs from a .json extension API via json_schema_compiler, types with no properties aren't handled well.

Example (from bookmark_manager_private.json):

   "types": [...
      {
        "id": "MetaInfoFields",
        "type": "object",
        "description": "Collection of meta info fields.",
        "additionalProperties": {"type": "string"}
      }...

Here, we don't specify any concrete properties on the MetaInfoFields type. It's just a bag of an arbitrary number of arbitrary properties. In other words, an {Object}.

(Note: additionalProperties means the type can include arbitrary properties, in this case of type string, but AFAICT, these are ignored in generating the externs.)

When it sees that MetaInfoFields has no properties, js_externs_generator.py skips adding the @typedef to the jsdoc comment:

    /**
     * Collection of meta info fields.
     * @see https://developer.chrome.com/extensions/bookmarkManagerPrivate#type-MetaInfoFields
     */
    chrome.bookmarkManagerPrivate.MetaInfoFields;

The result is a Closure error when other parts of the externs file reference MetaInfoFields:

  ## ../../../../third_party/closure_compiler/externs/bookmark_manager_private.js:160: ERROR - 
  Bad type annotation. Unknown type chrome.bookmarkManagerPrivate.MetaInfoFields
  ##  * @param {!chrome.bookmarkManagerPrivate.MetaInfoFields} metaInfo

Manually adding @typedef {Object} resolves the above error.