Don't run GCLH on advertisement iframes

On Firefox, for some reason, GCLH runs on the main window and also on each (i-)frame. This includes: Goodle ads, inline logging, and ads on the inline logging page. Hence, GCLH runs 2 or even 4 times per page.

This fix corrects this behavior such that GCLH runs only once, or twice with inline logging enabled. I've tested it on Firefox, Chrome, and Opera.