Add POINTER_WHEEL_CHANGED event to mojo.

ui/events/event.cc

Index: ui/events/event.cc
diff --git a/ui/events/event.cc b/ui/events/event.cc
index 60d09ef3ea0644c40793e5a280279a687c513944..69b44381b7206a23f09aa96b4e9a7fdafe79598d 100644
--- a/ui/events/event.cc
+++ b/ui/events/event.cc
@@ -662,6 +662,13 @@ MouseWheelEvent::MouseWheelEvent(const ScrollEvent& scroll_event)
   SetType(ET_MOUSEWHEEL);
 }
 
+MouseWheelEvent::MouseWheelEvent(const PointerEvent& pointer_event)
+    : MouseEvent(pointer_event),
+      offset_(pointer_event.pointer_details().offset.x(),
+              pointer_event.pointer_details().offset.y()) {
+  DCHECK(type() == ET_MOUSEWHEEL);
+}
+
 MouseWheelEvent::MouseWheelEvent(const MouseEvent& mouse_event,
                                  int x_offset,
                                  int y_offset)