[New Multicolumn] Support dynamic insertion and removal of multicol content.

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();
     void updateMarginProperties();
 
 protected:
     virtual void willBeRemovedFromTree() override;
     virtual bool needsPreferredWidthsRecalculation() const override;
     virtual void layout() override;
     virtual void computeLogicalHeight(LayoutUnit logicalHeight, LayoutUnit logicalTop, LogicalExtentComputedValues&) const override;
     virtual void invalidateTreeIfNeeded(const PaintInvalidationState&) override;
     virtual void paint(const PaintInfo&, const LayoutPoint& paintOffset) override;
     virtual bool nodeAtPoint(const HitTestRequest&, HitTestResult&, const HitTestLocation& locationInContainer, const LayoutPoint& accumulatedOffset, HitTestAction) 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