WebKit2 needs layoutTestController.overridePreference
*** Bug 58986 has been marked as a duplicate of this bug. ***
Support for overridePreference in WKTR was added for boolean values only in http://trac.webkit.org/changeset/125732 for https://bugs.webkit.org/show_bug.cgi?id=93942. This bug covers making it work for non-boolean values.
*** Bug 178779 has been marked as a duplicate of this bug. ***