Overscrolling and releasing should eventually settle at 0 offset

<a href="https://github.com/collinjackson"><img src="https://avatars.githubusercontent.com/u/394889?v=3" align="left" width="96" height="96" hspace="10"></img></a> **Issue by [collinjackson](https://github.com/collinjackson)**
_Friday Oct 09, 2015 at 22:08 GMT_
_Originally opened as https://github.com/flutter/engine/issues/1568_

---
