initWorld(transport, initializer) {
    this.sendToParent_ = transport(this.routeMessage_.bind(this, true));
    return new Promise(f => this.cookieCallback_ = f)
        .then(args => initializer ? initializer(...args) : undefined)
        .then(response => this.sendToParent_(
            {cookieResponse: true, worldId: this.worldId_, r: this.wrap_(response)}));
  }
Called in the child world to initialize it.
@param {!Transport} transport.
@param {function(...*):!Promise<*>} initializer