ga-create.js

// We assume that this is running in an iframe which loads a page from a domain
// that is specific to where your widget is served from. This means that for
// browsers that support 3rd party cookies in iframes, the cookie will be set
// on this domain, and for those who don't, localStorage will be used instead.
ga('create', 'UA-XXXX-Y', {
  'storage': 'none',
  'clientId', getPersistentVisitorId()
});