[performance-timeline] Performance Observer in Chrome Canary

Hey all. As a heads up, we have first* implementation of Performance
Observer in Chrome Canary.

To test it out boot up Chrome Canary and...
- Enable chrome://flags/#enable-experimental-web-platform-features
- Reboot Chrome Canary
- Visit demo page @ bit.ly/1OiRS7f

Any feedback would be much appreciated!

ig

* navigation timing is not yet hooked up to performance observer.