What it does, under the covers:

  (function() {
      var a = document.createElement("script");
      a.type = "text/javascript";
      a.innerText = 'window["_gaUserPrefs"] = { ioo : function() { return true; } }';
      document.documentElement.insertBefore(a, 
          document.documentElement.firstChild);
  })()

And it injects that into every site you visit:

  "matches": [
                "http://*/*",
                "https://*/*"
            ],
  

They should probably replace it with their declarative blocking functionality.