Make sure that no column rows get negative flow thread portion heights.

Source/core/layout/MultiColumnFragmentainerGroup.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef MultiColumnFragmentainerGroup_h
 #define MultiColumnFragmentainerGroup_h
 
 #include "core/layout/LayoutMultiColumnFlowThread.h"
 
 namespace blink {
@@ skipping 26 matching lines @@
     LayoutUnit logicalHeight() const { return m_columnHeight; }
 
     LayoutSize offsetFromColumnSet() const;
 
     // The top of our flow thread portion
     LayoutUnit logicalTopInFlowThread() const { return m_logicalTopInFlowThread; }
     void setLogicalTopInFlowThread(LayoutUnit logicalTopInFlowThread) { m_logicalTopInFlowThread = logicalTopInFlowThread; }
 
     // The bottom of our flow thread portion
     LayoutUnit logicalBottomInFlowThread() const { return m_logicalBottomInFlowThread; }
-    void setLogicalBottomInFlowThread(LayoutUnit logicalBottomInFlowThread) { m_logicalBottomInFlowThread = logicalBottomInFlowThread; }
+    void setLogicalBottomInFlowThread(LayoutUnit logicalBottomInFlowThread) { ASSERT(logicalBottomInFlowThread >= m_logicalTopInFlowThread); m_logicalBottomInFlowThread = logicalBottomInFlowThread; }
 
     // The height of our flow thread portion
     LayoutUnit logicalHeightInFlowThread() const { return m_logicalBottomInFlowThread - m_logicalTopInFlowThread; }
 
     bool heightIsAuto() const;
     void resetColumnHeight();
     void addContentRun(LayoutUnit endOffsetInFlowThread);
     void updateMinimumColumnHeight(LayoutUnit height) { m_minimumColumnHeight = std::max(height, m_minimumColumnHeight); }
     void recordSpaceShortage(LayoutUnit);
     bool recalculateColumnHeight(BalancedColumnHeightCalculation calculationMode);
@@ skipping 115 matching lines @@
 
 private:
     LayoutMultiColumnSet& m_columnSet;
 
     Vector<MultiColumnFragmentainerGroup, 1> m_groups;
 };
 
 } // namespace blink
 
 #endif // MultiColumnFragmentainerGroup_h