Avoid unnecessary descents into nested boxes with floats.

Source/core/rendering/RenderBlockFlow.cpp

 /*
  * Copyright (C) 2013 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:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 430 matching lines @@
     if (heightChanged)
         relayoutChildren = true;
 
     layoutPositionedObjects(relayoutChildren || isDocumentElement(), oldLeft != logicalLeft() ? ForcedLayoutAfterContainingBlockMoved : DefaultLayout);
 
     computeRegionRangeForBlock(flowThreadContainingBlock());
 
     // Add overflow from children (unless we're multi-column, since in that case all our child overflow is clipped anyway).
     computeOverflow(oldClientAfterEdge);
 
+    m_descendantsWithFloatsMarkedForLayout = false;
     return true;
 }
 
 void RenderBlockFlow::determineLogicalLeftPositionForChild(RenderBox* child, ApplyLayoutDeltaMode applyDelta)
 {
     LayoutUnit startPosition = borderStart() + paddingStart();
     if (style()->shouldPlaceBlockDirectionScrollbarOnLogicalLeft())
         startPosition -= verticalScrollbarWidth();
     LayoutUnit totalAvailableLogicalWidth = borderAndPaddingLogicalWidth() + availableLogicalWidth();
 
@@ skipping 1224 matching lines @@
     if (containsFloats())
         m_floatingObjects->clearLineBoxTreePointers();
     RenderBlock::deleteLineBoxTree();
 }
 
 void RenderBlockFlow::markAllDescendantsWithFloatsForLayout(RenderBox* floatToRemove, bool inLayout)
 {
     if (!everHadLayout() && !containsFloats())
         return;
 
+    if (m_descendantsWithFloatsMarkedForLayout && !floatToRemove)
+        return;
+    m_descendantsWithFloatsMarkedForLayout = m_descendantsWithFloatsMarkedForLayout || !floatToRemove;
+
     MarkingBehavior markParents = inLayout ? MarkOnlyThis : MarkContainingBlockChain;
     setChildNeedsLayout(markParents);
 
     if (floatToRemove)
         removeFloatingObject(floatToRemove);
 
     // Iterate over our children and mark them as needed.
     if (!childrenInline()) {
         for (RenderObject* child = firstChild(); child; child = child->nextSibling()) {
             if ((!floatToRemove && child->isFloatingOrOutOfFlowPositioned()) || !child->isRenderBlock())
@@ skipping 1092 matching lines @@
 RenderBlockFlow::RenderBlockFlowRareData& RenderBlockFlow::ensureRareData()
 {
     if (m_rareData)
         return *m_rareData;
 
     m_rareData = adoptPtr(new RenderBlockFlowRareData(this));
     return *m_rareData;
 }
 
 } // namespace WebCore