Hide Chrome Desktop tooltip if no milestone version

As requested first by @maxh, tooltip for "Chrome desktop" is hidden when no dekstop milestone version is set.

BUG=#122

![image](https://cloud.githubusercontent.com/assets/634478/3539642/76fae38a-083c-11e4-9e5e-e1b69cf1bd0c.png)

![screenshot from 2014-07-10 16 14 53](https://cloud.githubusercontent.com/assets/634478/3539685/b7420590-083c-11e4-8d42-2867b70774c8.png)