Resolve the Testcase Reducer iframe issue

[wisniewskit]

The Testcase Reducer addon does not work consistently across iframes, and perhaps due to other issues. I believe that the main issue here is https://bugzilla.mozilla.org/show_bug.cgi?id=1393022, and fixing it would greatly help with reducing complicated sites, increasing diagnosis throughput.

Rough time estimate: 2-4 days of effort, depending on how close the current patch is, unexpected test failures and the like.