Remove mainthread overhang painting code

Source/web/ScrollbarGroup.cpp

 /*
  * Copyright (C) 2011 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:
  *
  * 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
@@ skipping 203 matching lines @@
     if (m_verticalScrollbar)
         size.setHeight(m_verticalScrollbar->scrollbar()->totalSize());
     else if (m_horizontalScrollbar) {
         size.setHeight(m_horizontalScrollbar->scrollbar()->y());
         if (m_horizontalScrollbar->scrollbar()->isOverlayScrollbar())
             size.expand(0, WebPluginScrollbar::defaultThickness());
     }
     return size;
 }
 
-IntSize ScrollbarGroup::overhangAmount() const
-{
-    return IntSize();
-}
-
 IntPoint ScrollbarGroup::lastKnownMousePosition() const
 {
     return m_lastMousePosition;
 }
 
 bool ScrollbarGroup::shouldSuspendScrollAnimations() const
 {
     return false;
 }
 
@@ skipping 36 matching lines @@
     }
 
     int pageStep = std::max(
         static_cast<int>(static_cast<float>(length) * ScrollableArea::minFractionToStepWhenPaging()),
         length - ScrollableArea::maxOverlapBetweenPages());
 
     return std::max(pageStep, 1);
 }
 
 } // namespace blink