Avoid unnecessary descents into nested boxes with floats.

Source/core/rendering/RenderBlock.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2007 David Smith (catfish.man@gmail.com)
  * Copyright (C) 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010 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
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
@@ skipping 513 matching lines @@
         int m_lineBreakToAvoidWidow : 31;
         unsigned m_didBreakAtLineToAvoidWidow : 1;
      };
 
 protected:
     OwnPtr<RenderBlockRareData> m_rareData;
 
     RenderObjectChildList m_children;
     RenderLineBoxList m_lineBoxes;   // All of the root line boxes created for this block flow.  For example, <div>Hello<br>world.</div> will have two total lines for the <div>.
 
-    mutable signed m_lineHeight : 26;
+    mutable signed m_lineHeight : 25;
     unsigned m_hasMarginBeforeQuirk : 1; // Note these quirk values can't be put in RenderBlockRareData since they are set too frequently.
     unsigned m_hasMarginAfterQuirk : 1;
     unsigned m_beingDestroyed : 1;
     unsigned m_hasMarkupTruncation : 1;
     unsigned m_hasBorderOrPaddingLogicalWidthChanged : 1;
     mutable unsigned m_hasOnlySelfCollapsingChildren : 1;
+    mutable unsigned m_descendantsWithFloatsMarkedForLayout : 1;
 
     // RenderRubyBase objects need to be able to split and merge, moving their children around
     // (calling moveChildTo, moveAllChildrenTo, and makeChildrenNonInline).
     friend class RenderRubyBase;
     // FIXME-BLOCKFLOW: Remove this when the line layout stuff has all moved out of RenderBlock
     friend class LineBreaker;
 
     // FIXME: This is temporary as we move code that accesses block flow
     // member variables out of RenderBlock and into RenderBlockFlow.
     friend class RenderBlockFlow;
 };
 
 DEFINE_RENDER_OBJECT_TYPE_CASTS(RenderBlock, isRenderBlock());
 
 } // namespace WebCore
 
 #endif // RenderBlock_h