function getUid(obj) {
  return obj.ol_uid || (obj.ol_uid = String(++uidCounter_));
}
Gets a unique ID for an object. This mutates the object so that further calls
with the same object as a parameter returns the same value. Unique IDs are generated
as a strictly increasing sequence. Adapted from goog.getUid.

@param {Object} obj The object to get the unique ID for.
@return {string} The unique ID for the object.
@api