Cancel ongoing "fling" animations on tap-down

Synthesize WebGestureEvent::GestureFlingCancel on if we receive a
WebGestureEvent::GestureTapDown to stop the ongoing scroll animation.

Change-Id: I5b500ebbde815a54a466736492c2b5781f5ef91e
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>