[New Multicolumn] Add RenderMultiColumnSpannerSet.

Source/core/rendering/RenderMultiColumnSet.cpp

 /*
  * Copyright (C) 2012 Apple 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:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 188 matching lines @@
     if (!m_contentRuns.isEmpty() && endOffsetFromFirstPage <= m_contentRuns.last().breakOffset())
         return;
     // Append another item as long as we haven't exceeded used column count. What ends up in the
     // overflow area shouldn't affect column balancing.
     if (m_contentRuns.size() < usedColumnCount())
         m_contentRuns.append(ContentRun(endOffsetFromFirstPage));
 }
 
 bool RenderMultiColumnSet::recalculateColumnHeight(BalancedHeightCalculation calculationMode)
 {
+    if (previousSiblingMultiColumnSet()) {
+        // FIXME: column spanner layout is not yet implemented. Until it's in place, we only operate
+        // on the first set during layout. We need to ignore the others here, or assertions will
+        // fail.
+        return false;
+    }
     ASSERT(multiColumnFlowThread()->heightIsAuto());
 
     LayoutUnit oldColumnHeight = m_columnHeight;
     if (calculationMode == GuessFromFlowThreadPortion) {
         // Post-process the content runs and find out where the implicit breaks will occur.
         distributeImplicitBreaks();
     }
     LayoutUnit newColumnHeight = calculateColumnHeight(calculationMode);
     setAndConstrainColumnHeight(newColumnHeight);
 
@@ skipping 403 matching lines @@
 
 void RenderMultiColumnSet::detachRegion()
 {
     if (m_flowThread) {
         m_flowThread->removeRegionFromThread(this);
         m_flowThread = 0;
     }
 }
 
 }