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

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

 /*
  * Copyright (C) 2004, 2006, 2008 Apple 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 40 matching lines @@
       m_totalSize(0),
       m_currentPos(0),
       m_dragOrigin(0),
       m_hoveredPart(NoPart),
       m_pressedPart(NoPart),
       m_pressedPos(0),
       m_scrollPos(0),
       m_draggingDocument(false),
       m_documentDragPos(0),
       m_enabled(true),
-      m_scrollTimer(this, &Scrollbar::autoscrollTimerFired),
+      m_scrollTimer(scrollableArea->getTimerTaskRunner(),
+                    this,
+                    &Scrollbar::autoscrollTimerFired),
       m_elasticOverscroll(0),
       m_trackNeedsRepaint(true),
       m_thumbNeedsRepaint(true) {
   m_theme.registerScrollbar(*this);
 
   // FIXME: This is ugly and would not be necessary if we fix cross-platform
   // code to actually query for scrollbar thickness and use it when sizing
   // scrollbars (rather than leaving one dimension of the scrollbar alone when
   // sizing).
   int thickness = m_theme.scrollbarThickness(controlSize);
@@ skipping 561 matching lines @@
     invalidParts = AllParts;
   if (invalidParts & ~ThumbPart)
     m_trackNeedsRepaint = true;
   if (invalidParts & ThumbPart)
     m_thumbNeedsRepaint = true;
   if (m_scrollableArea)
     m_scrollableArea->setScrollbarNeedsPaintInvalidation(orientation());
 }
 
 }  // namespace blink