move getStats to adapter.js

So, cool idea to move this code behind the adapter.

However, let's move these changes to a separate file -- this adapter is published by the Chrome team and we periodically copy it into our codebase:
https://github.com/GoogleChrome/webrtc/blob/master/samples/web/js/adapter.js

If we start mixing in our own code here it'll become difficult to maintain. I'd suggest `src/webrtc/getstats.js`. We'll also have to add it to `uproxy-core-env.js` (see the Gruntfile for how this is built).

Perhaps we could also consider getting this added to `adapter.js`? :-)