Async Wheel Event with only the first one cancellable behind a flag.

content/browser/renderer_host/input/mouse_wheel_event_queue.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CONTENT_BROWSER_RENDERER_HOST_INPUT_MOUSE_WHEEL_EVENT_QUEUE_H_
 #define CONTENT_BROWSER_RENDERER_HOST_INPUT_MOUSE_WHEEL_EVENT_QUEUE_H_
 
 #include <deque>
 #include <memory>
 
@@ skipping 75 matching lines @@
   // True if a non-synthetic GSB needs to be sent before a GSU is sent.
   bool needs_scroll_begin_;
 
   // True if a non-synthetic GSE needs to be sent because a non-synthetic
   // GSB has been sent in the past.
   bool needs_scroll_end_;
 
   // True if the touchpad and wheel scroll latching is enabled.
   bool enable_scroll_latching_;
 
+  bool send_wheel_events_async_;
+
   blink::WebGestureDevice scrolling_device_;
 
   DISALLOW_COPY_AND_ASSIGN(MouseWheelEventQueue);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_RENDERER_HOST_INPUT_MOUSE_WHEEL_EVENT_QUEUE_H_