[New Multicolumn] Add RenderMultiColumnSpannerSet.

Source/core/rendering/RenderMultiColumnSet.cpp

Index: Source/core/rendering/RenderMultiColumnSet.cpp
diff --git a/Source/core/rendering/RenderMultiColumnSet.cpp b/Source/core/rendering/RenderMultiColumnSet.cpp
index efb34c280b22c0ba73502a33796ec30e7e58f589..f2451de20aa11709dfab46211f224c4a2514395a 100644
--- a/Source/core/rendering/RenderMultiColumnSet.cpp
+++ b/Source/core/rendering/RenderMultiColumnSet.cpp
@@ -206,6 +206,12 @@ void RenderMultiColumnSet::addContentRun(LayoutUnit 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;