Element hiding debug functionality causes hang in older Firefox

Background

adblockpluscore#220 affects as well element hiding emulation filters, but since it uses a different implementation of the hideElement() function this needs to be addressed seperatly.

What to change

Port the fix implemented in adblockpluscore#220 to the hideElement() function in adblockpluschrome.

Hints for testers

Make sure that when element hiding debugging is enabled, active element hiding emulation filters highlight the matching elements without causing excessive CPU load (or other undesired side effects) on Firefox 51-52.