[New Multicolumn] Let a spanner's containing block be the multicol container.

Source/core/rendering/RenderMultiColumnSpannerPlaceholder.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 RenderMultiColumnSpannerPlaceholder_h
+#define RenderMultiColumnSpannerPlaceholder_h
+
+#include "core/rendering/RenderMultiColumnFlowThread.h"
+
+namespace blink {
+
+// Placeholder renderer for column-span:all elements. The column-span:all renderer itself is a
+// descendant of the flow thread, but due to its out-of-flow nature, we need something on the
+// outside to take care of its positioning and sizing. RenderMultiColumnSpannerPlaceholder objects
+// are siblings of RenderMultiColumnSet objects, i.e. direct children of the multicol container.
+class RenderMultiColumnSpannerPlaceholder final : public RenderBox {
+public:
+    virtual bool isOfType(RenderObjectType type) const override { return type == RenderObjectRenderMultiColumnSpannerPlaceholder || RenderBox::isOfType(type); }
+
+    static RenderMultiColumnSpannerPlaceholder* createAnonymous(RenderStyle* parentStyle, RenderBox*);
+
+    RenderFlowThread* flowThread() const { return toRenderBlockFlow(parent())->multiColumnFlowThread(); }
+
+    RenderBox* rendererInFlowThread() const { return m_rendererInFlowThread; }
+    void setRendererInFlowThread(RenderBox* renderer) { m_rendererInFlowThread = renderer; }
+    void spannerWillBeRemoved();
+
+protected:
+    virtual void willBeRemovedFromTree() override;
+    virtual bool needsPreferredWidthsRecalculation() const override;
+    virtual void layout() override;
+    virtual void invalidateTreeIfNeeded(const PaintInvalidationState&) override;
+    virtual const char* renderName() const override;
+
+private:
+    RenderMultiColumnSpannerPlaceholder(RenderBox*);
+
+    RenderBox* m_rendererInFlowThread; // The actual column-span:all renderer inside the flow thread.
+};
+
+DEFINE_RENDER_OBJECT_TYPE_CASTS(RenderMultiColumnSpannerPlaceholder, isRenderMultiColumnSpannerPlaceholder());
+
+} // namespace blink
+
+#endif