getSignInOptions_() {
    const signInOptions = this.config_.get('signInOptions') || [];
    const normalizedOptions = [];
    for (let i = 0; i < signInOptions.length; i++) {
      const providerConfig = signInOptions[i];

      // If the config is not in object format, convert to object format.
      const normalizedConfig = goog.isObject(providerConfig) ?
          providerConfig : {'provider': providerConfig};

      if (normalizedConfig['provider']) {
        normalizedOptions.push(normalizedConfig);
      }
    }
    return normalizedOptions;
  }
Returns the normalized list of valid user-enabled IdPs.
The user may specify each IdP as just a provider ID or as an object
containing provider ID and additional scopes; this method converts all
entries to the object format and filters out entries with invalid
providers.
@return {!Array<?Object>} The normalized sign-in options.
@private