Support App + Web properties on Google Analytics (gtag)

[App + Web properties](https://blog.google/products/marketingplatform/analytics/new-way-unify-app-and-website-measurement-google-analytics/) is a new feature that Google Analytics that unifiies measurements on app and web properties.

Support this in gtag.js

cc/ @avimehta @zikas @zhouyx 