Implement Wheel Gesture Scrolling on OSX.

ui/events/blink/input_scroll_elasticity_controller.h

Index: ui/events/blink/input_scroll_elasticity_controller.h
diff --git a/ui/events/blink/input_scroll_elasticity_controller.h b/ui/events/blink/input_scroll_elasticity_controller.h
index 8c6468109c8e6df47efbf2565493b6c6999dfc5b..915b2e88af12cd7ee2e5a492e3a7f000f3f0c539 100644
--- a/ui/events/blink/input_scroll_elasticity_controller.h
+++ b/ui/events/blink/input_scroll_elasticity_controller.h
@@ -58,6 +58,15 @@ class InputScrollElasticityController {
   void ObserveWheelEventAndResult(
       const blink::WebMouseWheelEvent& wheel_event,
       const cc::InputHandlerScrollResult& scroll_result);
+  // Update the overscroll state based a gesture event that has been processed.
+  // Note that this assumes that all events are coming from a single input
+  // device. If the user simultaneously uses multiple input devices, Cocoa may
+  // not correctly pass all the gesture begin and end events. In this case,
+  // this class may disregard some scrolls that come in at unexpected times.
+  void ObserveGestureEventAndResult(
+      const blink::WebGestureEvent& gesture_event,
+      const cc::InputHandlerScrollResult& scroll_result);
+
   void Animate(base::TimeTicks time);
 
   void ReconcileStretchAndScroll();