Be a bit more efficient in .contentDestroyed().
Be a bit more efficient in .contentDestroyed().

Refs #1600

Martii/greasemonkey,the8472/greasemonkey,Ventero/greasemonkey,HasClass0/greasemonkey,Ventero/greasemonkey,the8472/greasemonkey,HasClass0/greasemonkey,Martii/greasemonkey