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..7198a5dedf9626699b330aa87fcd2bcbf53dee93 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_NE(ET_MOUSEWHEEL, type);
   latency()->AddLatencyNumber(INPUT_EVENT_LATENCY_UI_COMPONENT, 0, 0);
   if (this->type() == ET_MOUSE_MOVED && IsAnyButton())
     SetType(ET_MOUSE_DRAGGED);
@@ -679,13 +680,18 @@ 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) {
+  // Set event type to ET_UNKNOWN initially in MouseEvent() to pass the
+  // DCHECK for type to enforce that we use MouseWheelEvent() to create
+  // a MouseWheelEvent.
+  SetType(ui::ET_MOUSEWHEEL);
+}
 
 #if defined(OS_WIN)
 // This value matches windows WHEEL_DELTA.