Suppress "Uncaught ReferenceError: ext is not defined"

This error came started showing up in Chrome 37 and will
probably be fixed in Chrome 40 (https://crbug.com/416907).

References:
https://issues.adblockplus.org/ticket/1370
https://adblockplus.org/forum/viewtopic.php?f=10&t=25507
https://github.com/adblockplus/adblockpluschrome/pull/7