Remove frame flattening support as Chromium has no intention of using it

Source/core/platform/ScrollView.h

 /*
  * Copyright (C) 2004, 2006, 2007, 2008 Apple Inc. All rights reserved.
  * Copyright (C) 2009 Holger Hans Peter Freyther
  *
  * 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 80 matching lines @@
     void setHorizontalScrollbarLock(bool lock = true) { m_horizontalScrollbarLock = lock; }
     bool horizontalScrollbarLock() const { return m_horizontalScrollbarLock; }
     void setVerticalScrollbarLock(bool lock = true) { m_verticalScrollbarLock = lock; }
     bool verticalScrollbarLock() const { return m_verticalScrollbarLock; }
 
     void setScrollingModesLock(bool lock = true) { m_horizontalScrollbarLock = m_verticalScrollbarLock = lock; }
 
     virtual void setCanHaveScrollbars(bool);
     bool canHaveScrollbars() const { return horizontalScrollbarMode() != ScrollbarAlwaysOff || verticalScrollbarMode() != ScrollbarAlwaysOff; }
 
-    virtual bool avoidScrollbarCreation() const { return false; }
-
     virtual void setScrollbarOverlayStyle(ScrollbarOverlayStyle) OVERRIDE;
 
     // By default you only receive paint events for the area that is visible. In the case of using a
     // tiled backing store, this function can be set, so that the view paints the entire contents.
     bool paintsEntireContents() const { return m_paintsEntireContents; }
     void setPaintsEntireContents(bool);
 
     // By default, paint events are clipped to the visible area.  If set to
     // false, paint events are no longer clipped.  paintsEntireContents() implies !clipsRepaints().
     bool clipsRepaints() const { return m_clipsRepaints; }
@@ skipping 276 matching lines @@
     ASSERT(!widget || widget->isScrollView());
     return static_cast<const ScrollView*>(widget);
 }
 
 // This will catch anyone doing an unnecessary cast.
 void toScrollView(const ScrollView*);
 
 } // namespace WebCore
 
 #endif // ScrollView_h