IDBVersionChangeEvent is already used internally from non-Window
globals, but the constructor wasn't set up to expose the interface
object in worker globals.
Testing: The effort involved in adding a test for this does not seem
worthwhile—we get this for free once we enable the indexeddb preference
while our interfaces.worker.js test is running.
Fixes: part of #6963
Signed-off-by: Josh Matthews <josh@joshmatthews.net>