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

Source/core/layout/MultiColumnFragmentainerGroup.h

Index: Source/core/layout/MultiColumnFragmentainerGroup.h
diff --git a/Source/core/layout/MultiColumnFragmentainerGroup.h b/Source/core/layout/MultiColumnFragmentainerGroup.h
index c55994764c3d818caf258bc9eb9bd5b7fa67f5ce..96dee3bb4796b5fc4dec3f32df751af0f0612eae 100644
--- a/Source/core/layout/MultiColumnFragmentainerGroup.h
+++ b/Source/core/layout/MultiColumnFragmentainerGroup.h
@@ -44,7 +44,7 @@ public:
 
     // 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; }