Currently, sequenced smooth scroll only happens on main thread. To make it happen on the compositor thread, we need to send the kSequencedSmoothScroll type to the compositor thread together with the animation. This would help us distinguish SequencedSmoothScrolls and UserScrolls sent back from compositor thread. If it's a UserScroll, we should abort the all the sequenced scrolls in the queue, and keep the queue intact if it's a SequencedSmoothScrolls