[New Multicolumn] Add support for column-span:all

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/RenderBox.h"
+
+namespace blink {
+
+class RenderMultiColumnFlowThread;
+
+// An empty placeholder box inserted at the position in the render tree where the column-span:all
+// rendererer would naturally occur, had it not been column-span:all. A spanner box is moved from
+// its original position to become a sibling of the column sets. In other words it becomes a direct
+// child of the multicol container, since anything logically in the ancestry between the spanner and
+// the multicol container shouldn't affect the spanner (e.g. such ancestors should not affect the
+// size of the spanner, nor should their background be painted). The placeholder is used during flow
+// thread layout to figure out when to move from one column set to the next.
+class RenderMultiColumnSpannerPlaceholder FINAL : public RenderBox {
+public:
+    static RenderMultiColumnSpannerPlaceholder* createAnonymous(RenderMultiColumnFlowThread*, RenderBox* spanner, RenderStyle*);
+
+    virtual bool isRenderMultiColumnSpannerPlaceholder() const OVERRIDE { return true; }
+
+    // Get the actual spanner renderer; a sibling of the column sets.
+    RenderBox* spanner() const { return m_spanner; }
+
+    virtual void markForPaginationRelayoutIfNeeded(SubtreeLayoutScope&) OVERRIDE FINAL;
+    virtual void layout() OVERRIDE FINAL;
+
+private:
+    RenderMultiColumnSpannerPlaceholder(RenderMultiColumnFlowThread*, RenderBox* spanner);
+
+    virtual const char* renderName() const OVERRIDE;
+
+    RenderMultiColumnFlowThread* m_flowThread;
+    RenderBox* m_spanner;
+};
+
+DEFINE_RENDER_OBJECT_TYPE_CASTS(RenderMultiColumnSpannerPlaceholder, isRenderMultiColumnSpannerPlaceholder());
+
+} // namespace blink
+
+#endif // RenderMultiColumnSpannerPlaceholder_h