Create MouseWheelEvent directly instead of through MouseEvent.

ui/events/event.cc

Index: ui/events/event.cc
diff --git a/ui/events/event.cc b/ui/events/event.cc
index 8ce2ae6941c0fa6fe097d5ac418df2deedebd900..81c9abe66bc8efcd1355f624fecc8e39d3381afa 100644
--- a/ui/events/event.cc
+++ b/ui/events/event.cc
@@ -522,6 +522,7 @@ MouseEvent::MouseEvent(EventType type,
                    flags),
       changed_button_flags_(changed_button_flags),
       pointer_details_(PointerDetails(EventPointerType::POINTER_TYPE_MOUSE)) {
+  DCHECK(type != ET_MOUSEWHEEL);

[sadrul, 2016/08/29 16:02:36]

DCHECK_NE

[riajiang, 2016/08/29 16:17:54]

Done.

   latency()->AddLatencyNumber(INPUT_EVENT_LATENCY_UI_COMPONENT, 0, 0);
   if (this->type() == ET_MOUSE_MOVED && IsAnyButton())
     SetType(ET_MOUSE_DRAGGED);
@@ -679,13 +680,15 @@ MouseWheelEvent::MouseWheelEvent(const gfx::Vector2d& offset,
                                  base::TimeTicks time_stamp,
                                  int flags,
                                  int changed_button_flags)
-    : MouseEvent(ui::ET_MOUSEWHEEL,
+    : MouseEvent(ui::ET_UNKNOWN,
                  location,
                  root_location,
                  time_stamp,
                  flags,
                  changed_button_flags),
-      offset_(offset) {}
+      offset_(offset) {
+  SetType(ui::ET_MOUSEWHEEL);

[sadrul, 2016/08/29 16:02:36]

Add a comment here why we are doing this.

[riajiang, 2016/08/29 16:17:54]

Done.

+}
 
 #if defined(OS_WIN)
 // This value matches windows WHEEL_DELTA.