Stop canary from crashing

If you remove an iframe that has loaded an AppCache immediately after it has finished updating Chrome will pull a sad face:

![screen shot 2013-10-24 at 15 13 45](https://f.cloud.github.com/assets/825088/1399426/8ae5af62-3cb6-11e3-86e7-bbd3fa5373d0.png)

Adding a `setTimeout` (see this Pull Request) fixes it.