Write tests for privacy.thirdparty.isolate

We should write tests for testing DOM storage, image cache, and content cache isolation. I think that mochitests are the best way to go, as they allow you to fetch things from different domains: http://mxr.mozilla.org/mozilla-central/source/build/pgo/server-locations.txt

I think all of the stuff behind network.thirdparty.isolate should be our main priority of things to merge with Mozilla. Historically, these components also have a high rate of breaking between releases, so we should get this coverage as soon as possible.