function sessionStorageSize() {
  return this.requestHandler.create('/session/:sessionId/session_storage/size');
}
Protocol bindings to get the session storage size. (Not part of the official
Webdriver specification).

@return {Number} The number of items in the storage.

@see  https://github.com/SeleniumHQ/selenium/wiki/JsonWireProtocol#sessionsessionidsession_storagesize
@type protocol