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

Source/core/rendering/RenderFlowThread.h

Index: Source/core/rendering/RenderFlowThread.h
diff --git a/Source/core/rendering/RenderFlowThread.h b/Source/core/rendering/RenderFlowThread.h
index 9b7fd98c9a28e9c520331ad0a3cf30a98fd8d258..f46d54194aca3527d1c228b57ba6190ab294cc8b 100644
--- a/Source/core/rendering/RenderFlowThread.h
+++ b/Source/core/rendering/RenderFlowThread.h
@@ -67,6 +67,17 @@ public:
     // can easily avoid drawing the children directly.
     virtual LayerType layerTypeRequired() const OVERRIDE FINAL { return NormalLayer; }
 
+    // Some renderers (column spanners) are moved out of the flow thread to live among column
+    // sets. If |child| is such a renderer, resolve it to the placeholder that lives at the original
+    // location in the tree.
+    virtual RenderObject* resolveMovedChild(RenderObject* child) const { return child; }
+    // Called when a descendant of the flow thread has been inserted.
+    virtual void flowThreadDescendantInserted(RenderObject*) { }
+    // Called when a sibling or descendant of the flow thread is about to be removed.
+    virtual void flowThreadRelativeWillBeRemoved(RenderObject*) { }
+    // Called when a descendant box's layout is finished and it has been positioned within its container.
+    virtual void flowThreadDescendantBoxLaidOut(RenderBox*, LayoutUnit /*logicalBottomInFlowThread*/) { }
+
     virtual bool nodeAtPoint(const HitTestRequest&, HitTestResult&, const HitTestLocation& locationInContainer, const LayoutPoint& accumulatedOffset, HitTestAction) OVERRIDE FINAL;
 
     virtual void addRegionToThread(RenderRegion*) = 0;