Re-target wheel events only when a new scroll sequence has started.

third_party/WebKit/Source/core/input/EventHandler.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011 Apple Inc. All rights
  * reserved.
  * Copyright (C) 2006 Alexey Proskuryakov (ap@webkit.org)
  * Copyright (C) 2012 Digia Plc. and/or its subsidiary(-ies)
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
@@ skipping 220 matching lines @@
   last_scrollbar_under_mouse_ = nullptr;
   frame_set_being_resized_ = nullptr;
   drag_target_ = nullptr;
   should_only_fire_drag_over_event_ = false;
   last_mouse_down_user_gesture_token_.Clear();
   capturing_mouse_events_node_ = nullptr;
   pointer_event_manager_->Clear();
   scroll_manager_->Clear();
   gesture_manager_->Clear();
   mouse_event_manager_->Clear();
+  mouse_wheel_event_manager_->Clear();
   last_deferred_tap_element_ = nullptr;
   event_handler_will_reset_capturing_mouse_events_node_ = false;
 }
 
 void EventHandler::UpdateSelectionForMouseDrag() {
   mouse_event_manager_->UpdateSelectionForMouseDrag();
 }
 
 void EventHandler::StartMiddleClickAutoscroll(LayoutObject* layout_object) {
   DCHECK(RuntimeEnabledFeatures::MiddleClickAutoscrollEnabled());
@@ skipping 1820 matching lines @@
     MouseEventWithHitTestResults& mev,
     LocalFrame* subframe) {
   WebInputEventResult result =
       subframe->GetEventHandler().HandleMouseReleaseEvent(mev.Event());
   if (result != WebInputEventResult::kNotHandled)
     return result;
   return WebInputEventResult::kHandledSystem;
 }
 
 }  // namespace blink