Bug 887978 - Pref on WebVTT for non-release builds. r=cpearce, bz

This allows us to get wider testing without having to worry
about fliping the pref back off for release if we find problems.

New global interafaces need to be added to test_interfaces.html
and marked with the same conditional as the pref. The test must
also be updated if the conditional is changed on the pref.