Fallback to old WebView where SFSafariViewController is not supported
function WebView() {
  this.tab = null;
  this.handler = null;
  this.open = this.open.bind(this);
  this.handleFirstLoadEnd = this.handleFirstLoadEnd.bind(this);
  this.handleLoadError = this.handleLoadError.bind(this);
  this.handleExit = this.handleExit.bind(this);
  this.clearEvents = this.clearEvents.bind(this);
  this.close = this.close.bind(this);
}