Avoid unnecessary descents into nested boxes with floats.

Source/core/rendering/RenderBlockFlow.cpp

Index: Source/core/rendering/RenderBlockFlow.cpp
diff --git a/Source/core/rendering/RenderBlockFlow.cpp b/Source/core/rendering/RenderBlockFlow.cpp
index 5d7a47b8a5c040eb278d374d3e19402d31023d5a..14fe9a5b6fe5f2c58fdd9f5978da6eebc97bc3e3 100644
--- a/Source/core/rendering/RenderBlockFlow.cpp
+++ b/Source/core/rendering/RenderBlockFlow.cpp
@@ -452,6 +452,7 @@ inline bool RenderBlockFlow::layoutBlockFlow(bool relayoutChildren, LayoutUnit &
     // 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;
 }
 
@@ -1696,6 +1697,10 @@ void RenderBlockFlow::markAllDescendantsWithFloatsForLayout(RenderBox* floatToRe
     if (!everHadLayout() && !containsFloats())
         return;
 
+    if (m_descendantsWithFloatsMarkedForLayout && !floatToRemove)
+        return;
+    m_descendantsWithFloatsMarkedForLayout = !m_descendantsWithFloatsMarkedForLayout && !floatToRemove;

[Julien - ping for review, 2014/04/23 00:46:21]

The old code would just mark m_descendantWithFloatsMarkedForLayout to true. In
this code, you could potentially flip the value of the boolean from true to
false (because floatToRemove is true). Is that really intended? If so, I would
need some comment explaining the details as I have a hard time understand the
boolean logic on this line.

+
     MarkingBehavior markParents = inLayout ? MarkOnlyThis : MarkContainingBlockChain;
     setChildNeedsLayout(markParents);