A better way to reference the global window.ga

Created by: wbazant

The previous PR was my mistake and I clearly didn't test enough, sorry!

Instead of "catch when ga is undefined" reference the global object from window

You can't use an undefined global - that's the scenario for Uncaught ReferenceError - unless by seeing if it's present in window (says the Internet)

mitigates ebiwd#96

This reverts commit 3286d81c