Prevent SET_SCROLL_OFFSET spam from clogging tubes

 Bug: 3393571
 Limit the event queue to a single pending SET_SCROLL_OFFSET. The
 problem on slashdot.org was that the event thread was being flooded
 with SET_SCROLL_OFFSET messages faster than it could process them,
 causing the handler to get jammed up.

Change-Id: Ia4a90a030959d0b3769067791089cdb261ae403a