Enable the DOM agent so Valence can start properly with Chrome canary

Fixes issue #94 which prevented debugging chrome canary (I think > 39, but it could be 40).
Something changed in the chrome protocol so that it's now mandatory to enable the DOM agent (requesting `"CSS.enable"` complained about it).
I added the code to root.js because that's where the CSS Sheet store is initialized, and we also need the DOM agent to be enabled in the walker, so I figured let's enable it once and for all from the start.
The whole thing is try/catched because the call is only necessary on canary and fails on other devices/browsers.