HTML5 canvas inspector?

EDIT:
this answer doesn’t work on new chrome versions see:
chrome canvas inspector 2015

In Chrome Canary:

1. in your browser, enter this url chrome://flags/
2. enable Enable Developer Tools experiments
3. relaunch Chrome
4. in the developer tools, click the gear to bring up developer preferences
5. select experiments from the menu
6. select Canvas Inspections
7. close devtools, refresh the page, reopen devtools

The full guide on the canvas profiler:
http://www.html5rocks.com/en/tutorials/canvas/inspection/

Animated gif showing it in action:
https://twitter.com/umaar/status/480705624448045057