Added missing import for filterClasses.ActiveFilter to the background page

This line was removed with https://hg.adblockplus.org/adblockpluschrome/rev/7d2cd26dabb5 for no obvious reason, leading to reference errors in the background page.

[Thomas Greiner, 2014-04-09 10:09:34]

It's no longer needed in background.js because the initAntiAdblockNotification
was moved to the adblockplus subrepo.

The reference error you mention is taken care of in
https://hg.adblockplus.org/adblockplus/rev/7eb29729a72c

[Sebastian Noack, 2014-04-09 10:09:57]

Apparently this was already fixed inside the adblockplus repository
(https://hg.adblockplus.org/adblockplus/rev/7eb29729a72c). However it seems that
we can't update the subrepo yet, due to
https://issues.adblockplus.org/ticket/189. :(

So unless somebody has a better idea, I would like to merge this patch, adding a
comment that the line can be removed again later, when the subrepo was updated.

[Wladimir Palant, 2014-04-09 10:11:27]

LGTM

[Thomas Greiner, 2014-04-09 10:14:12]

> So unless somebody has a better idea, I would like to merge this patch, adding
a
> comment that the line can be removed again later, when the subrepo was
updated.

LGTM with that addition.