[New Multicolumn] Add RenderMultiColumnSpannerSet.

Source/core/rendering/RenderMultiColumnSpannerSet.h

+// Copyright 2014 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 RenderMultiColumnSpannerSet_h
+#define RenderMultiColumnSpannerSet_h
+
+#include "core/rendering/RenderMultiColumnSet.h"
+
+namespace blink {
+
+// "Column" set for column-span:all elements. Positions the portion of the flow thread that a
+// spanner takes up. The actual renderer that sets column-span all is laid out as part of flow
+// thread layout (although it cannot use the flow thread width when calculating its own with; it has
+// to pretend that the containing block is the multicol container, not the flow
+// thread). RenderMultiColumnSpannerSet just makes sure that it's positioned correctly among other
+// column sets, within the multicol container.
+//
+// FIXME: should consider not inheriting from RenderMultiColumnSet (but rather from some common base
+// class). Most of what's in RenderMultiColumnSet is of no interest to us.
+class RenderMultiColumnSpannerSet final : public RenderMultiColumnSet {
+public:
+    virtual bool isOfType(RenderObjectType type) const override { return type == RenderObjectRenderMultiColumnSpannerSet || RenderMultiColumnSet::isOfType(type); }
+
+    static RenderMultiColumnSpannerSet* createAnonymous(RenderMultiColumnFlowThread*, RenderStyle* parentStyle, RenderBox*);
+
+    RenderBox* rendererInFlowThread() const { return m_rendererInFlowThread; }
+
+protected:
+    virtual bool recalculateColumnHeight(BalancedHeightCalculation) override;
+    virtual const char* renderName() const override;
+
+private:
+    RenderMultiColumnSpannerSet(RenderMultiColumnFlowThread*, RenderBox*);
+
+    RenderBox* m_rendererInFlowThread; // The actual column-span:all renderer inside the flow thread.
+};
+
+DEFINE_RENDER_OBJECT_TYPE_CASTS(RenderMultiColumnSpannerSet, isRenderMultiColumnSpannerSet());
+
+} // namespace blink
+
+#endif