Backed out changeset 13c1b3a2d135 (bug 880625) for xpcshell failures.
Bug 887978 - Enable WebVTT. r=cpearce, bz
New global interfaces 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.

We now pass the track element portion of the imptests.