Implement lazy block layout prototype behind a runtime flag.

Source/core/page/FrameView.h

 /*
    Copyright (C) 1997 Martin Jones (mjones@kde.org)
              (C) 1998 Waldo Bastian (bastian@kde.org)
              (C) 1998, 1999 Torben Weis (weis@kde.org)
              (C) 1999 Lars Knoll (knoll@kde.org)
              (C) 1999 Antti Koivisto (koivisto@kde.org)
    Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009 Apple Inc. All rights reserved.
 
    This library is free software; you can redistribute it and/or
    modify it under the terms of the GNU Library General Public
@@ skipping 86 matching lines @@
 
     void layout(bool allowSubtree = true);
     bool didFirstLayout() const;
     void layoutTimerFired(Timer<FrameView>*);
     void scheduleRelayout();
     void scheduleRelayoutOfSubtree(RenderObject*);
     void unscheduleRelayout();
     bool layoutPending() const;
     bool isInLayout() const { return m_inLayout; }
 
+    void layoutLazyBlocks();
+
     RenderObject* layoutRoot(bool onlyDuringLayout = false) const;
     void clearLayoutRoot() { m_layoutRoot = 0; }
     int layoutCount() const { return m_layoutCount; }
 
     bool needsLayout() const;
     void setNeedsLayout();
     void setViewportConstrainedObjectsNeedLayout();
 
     bool needsFullRepaint() const { return m_doFullRepaint; }
 
@@ skipping 474 matching lines @@
     ASSERT(!widget || widget->isFrameView());
     return static_cast<const FrameView*>(widget);
 }
 
 // This will catch anyone doing an unnecessary cast.
 void toFrameView(const FrameView*);
 
 } // namespace WebCore
 
 #endif // FrameView_h