Articulate the cancel behavior in WebMouseWheelEvent.

third_party/WebKit/public/web/WebInputEvent.h

 /*
  * Copyright (C) 2009 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 430 matching lines @@
     bool hasPreciseScrollingDeltas;
 
     // When false, this wheel event should not trigger scrolling (or any other default
     // action) if the event goes unhandled by JavaScript. This is used, for example,
     // when the browser decides the default behavior for Ctrl+Wheel should be to zoom
     // instead of scroll.
     bool canScroll;
 
     RailsMode railsMode;
 
+    // Whether the event is blocking, non-blocking, all event
+    // listeners were passive or was forced to be non-blocking.
+    DispatchType dispatchType;
+
     WebMouseWheelEvent()
         : WebMouseEvent(sizeof(WebMouseWheelEvent))
         , deltaX(0.0f)
         , deltaY(0.0f)
         , wheelTicksX(0.0f)
         , wheelTicksY(0.0f)
         , accelerationRatioX(1.0f)
         , accelerationRatioY(1.0f)
         , resendingPluginId(-1)
         , phase(PhaseNone)
         , momentumPhase(PhaseNone)
         , canRubberbandLeft(true)
         , canRubberbandRight(true)
         , scrollByPage(false)
         , hasPreciseScrollingDeltas(false)
         , canScroll(true)
         , railsMode(RailsModeFree)
+        , dispatchType(Blocking)
     {
     }
 };
 
 // WebGestureEvent --------------------------------------------------------------
 
 class WebGestureEvent : public WebInputEvent {
 public:
     enum ScrollUnits {
         PrecisePixels = 0, // generated by high precision devices.
@@ skipping 161 matching lines @@
         , uniqueTouchEventId(0)
     {
     }
 };
 
 #pragma pack(pop)
 
 } // namespace blink
 
 #endif