Add support for native font load events.

*Needs review.* First stab at supporting the native font load events in Chrome Canary (http://dev.w3.org/csswg/css-font-loading/). Also see #179.
It is arbitrary and one of the oddities of the native font load API. You need to pass a valid font shorthand (e.g. `16px <family>`) for it to work. The actual size is ignored. We use 300px to test for font loads elsewhere, so I just stuck with it.