Fix chrome extension build

The chrome extension build was broken by f7bcdc0b6a001f678bb446d51127a4eb769e8521, which made the `service_url` absolute without checking (as the `base_url` context item does) to see if `request.root` was present.

As the extension only appears to use the linked assets, and not the template HTML, this change simply doesn't emit a service URL when `request.root` is unset.

In order to prevent this kind of regression in future, I've also added the Chrome build to Travis.