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

Source/core/rendering/RenderFlowThread.h

 /*
  * Copyright (C) 2011 Adobe Systems Incorporated. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1. Redistributions of source code must retain the above
  *    copyright notice, this list of conditions and the following
  *    disclaimer.
@@ skipping 47 matching lines @@
 
     virtual bool supportsPaintInvalidationStateCachedOffsets() const override { return false; }
 
     virtual void layout() override;
 
     // Always create a RenderLayer for the RenderFlowThread so that we
     // can easily avoid drawing the children directly.
     virtual LayerType layerTypeRequired() const override final { return NormalLayer; }
 
     virtual void flowThreadDescendantWasInserted(RenderObject*) { }
+    virtual void flowThreadDescendantWillBeRemoved(RenderObject*) { }
 
     virtual bool nodeAtPoint(const HitTestRequest&, HitTestResult&, const HitTestLocation& locationInContainer, const LayoutPoint& accumulatedOffset, HitTestAction) override final;
 
     virtual void addRegionToThread(RenderMultiColumnSet*) = 0;
     virtual void removeRegionFromThread(RenderMultiColumnSet*);
 
     virtual void computeLogicalHeight(LayoutUnit logicalHeight, LayoutUnit logicalTop, LogicalExtentComputedValues&) const override;
 
     bool hasRegions() const { return m_multiColumnSetList.size(); }
 
@@ skipping 95 matching lines @@
 };
 
 template <> struct ValueToString<RenderMultiColumnSet*> {
     static String string(const RenderMultiColumnSet* value) { return String::format("%p", value); }
 };
 #endif
 
 } // namespace blink
 
 #endif // RenderFlowThread_h