[New Multicolumn] balancer confused by content with top margins.

Source/core/rendering/RenderFlowThread.h

 /*
  * Copyright (C) 2011 Adobe Systems Incorporated. All rights reserved.
  *
  * 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.
@@ skipping 100 matching lines @@
 
     bool previousRegionCountChanged() const { return m_previousRegionCount != m_regionList.size(); }
     void updatePreviousRegionCount() { m_previousRegionCount = m_regionList.size(); }
 
     void setRegionRangeForBox(const RenderBox*, LayoutUnit offsetFromLogicalTopOfFirstPage);
     void getRegionRangeForBox(const RenderBox*, RenderRegion*& startRegion, RenderRegion*& endRegion) const;
 
     virtual bool addForcedRegionBreak(LayoutUnit, RenderObject* breakChild, bool isBefore, LayoutUnit* offsetBreakAdjustment = 0) { return false; }
     void applyBreakAfterContent(LayoutUnit);
 
+    virtual bool isPageLogicalHeightKnown() const { return true; }
     bool pageLogicalSizeChanged() const { return m_pageLogicalSizeChanged; }
 
     void collectLayerFragments(LayerFragments&, const LayoutRect& layerBoundingBox, const LayoutRect& dirtyRect);
     LayoutRect fragmentsBoundingBox(const LayoutRect& layerBoundingBox);
 
     void pushFlowThreadLayoutState(const RenderObject&);
     void popFlowThreadLayoutState();
     LayoutUnit offsetFromLogicalTopOfFirstRegion(const RenderBlock*) const;
 
     // Used to estimate the maximum height of the flow thread.
@@ skipping 110 matching lines @@
 };
 
 template <> struct ValueToString<RenderRegion*> {
     static String string(const RenderRegion* value) { return String::format("%p", value); }
 };
 #endif
 
 } // namespace WebCore
 
 #endif // RenderFlowThread_h