Mock a the minimal subset of WebRTC for testing.

The intent of this PR is to introduce a basic mocking of the WebRTC API. Just enough to make [shortwave](inetCatapult/shortwave) work with Zombie.js integration tests.

I am going to be adding tests to this PR.
I don't understand why we want to add the zombie mock APIs to the global browser scope like this. Could you explain your reasoning here?