[Pickers] Step doesn't work

## Expected Behavior
https://material-ui-1dab0.firebaseapp.com/demos/pickers/#date-amp-time-pickers
This picker should step time every 300 secondes but it doesn't on Google Chrome.
Even though I change to 900 for 15 minutes step, nothing happens.

But it works here:
http://jsfiddle.net/MkxHE/

| Tech         | Version |
|--------------|---------|
| Material-UI  |  v1.0.0-beta.12 |