Fix scrolling rails on Mac OSX.

content/browser/renderer_host/input/mouse_wheel_event_queue.cc

Index: content/browser/renderer_host/input/mouse_wheel_event_queue.cc
diff --git a/content/browser/renderer_host/input/mouse_wheel_event_queue.cc b/content/browser/renderer_host/input/mouse_wheel_event_queue.cc
index 5e56e9b821ec1e90d46749752c8dac920b5efe7f..2004c1677f4905e2ba8047a9ffbc05d7cbefb934 100644
--- a/content/browser/renderer_host/input/mouse_wheel_event_queue.cc
+++ b/content/browser/renderer_host/input/mouse_wheel_event_queue.cc
@@ -122,6 +122,13 @@ void MouseWheelEventQueue::ProcessMouseWheelAck(
           event_sent_for_gesture_ack_->event.hasPreciseScrollingDeltas
               ? blink::WebGestureEvent::PrecisePixels
               : blink::WebGestureEvent::Pixels;
+
+      if (event_sent_for_gesture_ack_->event.railsMode ==
+          WebInputEvent::RailsModeVertical)
+        scroll_update.event.data.scrollUpdate.deltaX = 0;
+      if (event_sent_for_gesture_ack_->event.railsMode ==
+          WebInputEvent::RailsModeHorizontal)
+        scroll_update.event.data.scrollUpdate.deltaY = 0;
     }
 
     bool current_phase_ended = false;