Intermittent timeout in /_mozilla/css/img_dynamic_remove.html

```
  ▶ TIMEOUT [expected PASS] /_mozilla/css/img_dynamic_remove.html
  │ 
  │ VMware, Inc.
  │ Gallium 0.4 on softpipe
  │ 3.3 (Core Profile) Mesa 12.0.1
  └ Shutting down the Constellation after generating an output file or exit flag specified
```