Before this changeset, mochitests in browser/base/content/test/webrtc will pick
up loopback devices due to the changes made earlier in this patch chain to
device enumeration. This results in test bustages (for unknown reasons).
This changeset updates the setup JS for these tests to make sure loopback
devices prefs are unset. A comment has been added noting that if the tests
can be made to work with loopback devices that would be ideal.
MozReview-Commit-ID: KG0io8s5r86