Initial support for nested multicol layout.

Source/core/layout/LayoutMultiColumnSet.h

Index: Source/core/layout/LayoutMultiColumnSet.h
diff --git a/Source/core/layout/LayoutMultiColumnSet.h b/Source/core/layout/LayoutMultiColumnSet.h
index 6397fb0bf55aa8377e92094a2de786f5eeb874cd..b7c28b9f483c9e3ca42d774bee5802cd08a2a839 100644
--- a/Source/core/layout/LayoutMultiColumnSet.h
+++ b/Source/core/layout/LayoutMultiColumnSet.h
@@ -64,8 +64,14 @@ public:
 
     const MultiColumnFragmentainerGroup& firstFragmentainerGroup() const { return m_fragmentainerGroups.first(); }
     const MultiColumnFragmentainerGroup& lastFragmentainerGroup() const { return m_fragmentainerGroups.last(); }
-    MultiColumnFragmentainerGroup& fragmentainerGroupAtFlowThreadOffset(LayoutUnit);
-    const MultiColumnFragmentainerGroup& fragmentainerGroupAtFlowThreadOffset(LayoutUnit) const;
+    MultiColumnFragmentainerGroup& fragmentainerGroupAtFlowThreadOffset(LayoutUnit flowThreadOffset)
+    {
+        return m_fragmentainerGroups[fragmentainerGroupIndexAtFlowThreadOffset(flowThreadOffset)];
+    }
+    const MultiColumnFragmentainerGroup& fragmentainerGroupAtFlowThreadOffset(LayoutUnit flowThreadOffset) const
+    {
+        return m_fragmentainerGroups[fragmentainerGroupIndexAtFlowThreadOffset(flowThreadOffset)];
+    }
     const MultiColumnFragmentainerGroup& fragmentainerGroupAtVisualPoint(const LayoutPoint&) const;
 
     bool isOfType(LayoutObjectType type) const override { return type == LayoutObjectLayoutMultiColumnSet || LayoutBlockFlow::isOfType(type); }
@@ -85,6 +91,8 @@ public:
     LayoutMultiColumnSet* nextSiblingMultiColumnSet() const;
     LayoutMultiColumnSet* previousSiblingMultiColumnSet() const;
 
+    MultiColumnFragmentainerGroup& appendNewFragmentainerGroup();
+
     LayoutUnit logicalTopInFlowThread() const;
     LayoutUnit logicalBottomInFlowThread() const;
     LayoutUnit logicalHeightInFlowThread() const { return logicalBottomInFlowThread() - logicalTopInFlowThread(); }
@@ -158,6 +166,8 @@ protected:
     LayoutMultiColumnSet(LayoutFlowThread*);
 
 private:
+    unsigned fragmentainerGroupIndexAtFlowThreadOffset(LayoutUnit) const;
+
     void insertedIntoTree() final;
     void willBeRemovedFromTree() final;