Investigate whether blocked element hiding can kick in earlier

[gorhill]

Test case:

• http://www.theglobeandmail.com/

The banner at the top can be seen before it disappear. The banner is blocked through a net request filter, thus it's the blocked element hider code taking care to collapse the banner (need to confirm completely cosmetic filters have nothing to do there).

[gorhill]

Not caused by element hider, it was the case of a generic cosmetic filter not applying early enough (due to genericity). The fact that the banner was at the top was making the problem more noticeable than usual.