Make it possible to use preferences to configure webrender and titlebar
It's getting tiring for browserhtml to run servo with so many options:
`servo -w -b --pref dom.mozbrowser.enabled --pref dom.forcetouch.enabled http://localhost:6060`.
We want to be able to control all of these with preferences (we have a custom pref file).
Only webrender and titlebar are not controllable via a pref. This PR makes it possible.
<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="35" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/10336)
<!-- Reviewable:end -->