[whatwg] Outline style to use for drawSystemFocusRing

[Dominic Mazzoni]

We've finished implementing drawSystemFocusRing and drawCustomFocusRing in
Chrome. Try it in Chrome 31 or higher (either canary or dev channel should
work today). You'll need to go to chrome://flags and enable *experimental
canvas features*, then restart the browser.

Here's a demo I built that uses drawSystemFocusRing:
http://dmazzoni-google.github.io/canvas-focus-ring-demo/

- Dominic