Fix an issue with flatten transform that surfaced with the latest Chrome

We noticed that line marks stopped rendering on the latest Chrome (v38.0.2125.101).

Example JSON: https://gist.github.com/yongjunj/bd16feb86717744e96e1

Issue seems to be the `flatten` data transform we were applying at the marks group level.
More specifically:

FF v32.0.3: `data.values === undefined`
![screen shot 2014-10-10 at 12_45_33 pm](https://cloud.githubusercontent.com/assets/1096017/4599903/5cf6641c-50cd-11e4-999d-bcf20c3bf5a2.png)

Chrome v38.0.2125.101: `data.values !== undefined`
![screen shot 2014-10-10 at 12_45_27 pm](https://cloud.githubusercontent.com/assets/1096017/4599907/60ed96a8-50cd-11e4-99b0-01728a985285.png)

Here's a proposed fix. I've confirmed all our visualizations as well as Vega online editor examples render correctly