Enable the spinner on mobile webkit, it works now.

issue #3342
Fix spinner rotation for older Android's

* Instead of using "steps" for the animation timing, use linear.
* Instead of using "turn" for the animation from/to, use degrees.
Merge pull request #3416 from mozilla/issue_3342_webkit_spinner

Enable the spinner on mobile webkit, it works now.

Merging. This is for marketplace. Thanks @6a68.