Have Scrollbar's timer use the same task runner as its scrollable area.

third_party/WebKit/Source/platform/scroll/Scrollbar.h

 /*
  * Copyright (C) 2004, 2006 Apple Computer, 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:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 218 matching lines @@
 
   ScrollbarPart m_hoveredPart;
   ScrollbarPart m_pressedPart;
   int m_pressedPos;
   float m_scrollPos;
   bool m_draggingDocument;
   int m_documentDragPos;
 
   bool m_enabled;
 
-  Timer<Scrollbar> m_scrollTimer;
+  TaskRunnerTimer<Scrollbar> m_scrollTimer;
 
   float m_elasticOverscroll;
 
  private:
   bool isScrollbar() const override { return true; }
 
   void invalidate() override { setNeedsPaintInvalidation(AllParts); }
   void invalidateRect(const IntRect&) override {
     setNeedsPaintInvalidation(AllParts);
   }
 
   float scrollableAreaCurrentPos() const;
   float scrollableAreaTargetPos() const;
   bool thumbWillBeUnderMouse() const;
 
   int m_themeScrollbarThickness;
   bool m_trackNeedsRepaint;
   bool m_thumbNeedsRepaint;
 };
 
 DEFINE_TYPE_CASTS(Scrollbar,
                   Widget,
                   widget,
                   widget->isScrollbar(),
                   widget.isScrollbar());
 
 }  // namespace blink
 
 #endif  // Scrollbar_h