For developing properties, we will handle them in an other bug.
Besides, I use an iframe for the test because we create a use counter in
the constructor of Document, which use the prefs to decide what kind of
properties we want to record. So, in the test, we have to reload iframe
to make sure we re-create the document, so does the use counter, to make
sure the prefs work properly.
The two prefs affect the css use counters:
1. layout.css.use-counters.enabled: Allocate use counters, and record
non-custom properties.
2. layout.css.use-counters-unimplemented.enabled: Record all unimplmented
properties into the use counters.
If we disable layout.css.use-counters.enblaed, we don't create use counters
object, so layout.css.use-counters-unimplemented.enabled doesn't work,
either.
Differential Revision: https://phabricator.services.mozilla.com/D43860
