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

Source/core/rendering/RenderObject.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2000 Dirk Mueller (mueller@kde.org)
  *           (C) 2004 Allan Sandfeld Jensen (kde@carewolf.com)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2009 Google Inc. All rights reserved.
  * Copyright (C) 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  *
  * This library is free software; you can redistribute it and/or
@@ skipping 2382 matching lines @@
 
 void RenderObject::removeFromRenderFlowThread()
 {
     if (flowThreadState() == NotInsideFlowThread)
         return;
 
     // Sometimes we remove the element from the flow, but it's not destroyed at that time.
     // It's only until later when we actually destroy it and remove all the children from it.
     // Currently, that happens for firstLetter elements and list markers.
     // Pass in the flow thread so that we don't have to look it up for all the children.
-    removeFromRenderFlowThreadRecursive(flowThreadContainingBlock());
+    // If we're a column spanner, we need to use our parent to find the flow thread, since a spanner
+    // doesn't have the flow thread in its containing block chain. We still need to notify the flow
+    // thread when the renderer removed happens to be a spanner, so that we get rid of the spanner
+    // placeholder, and column sets around the placeholder get merged.
+    RenderFlowThread* flowThread = isColumnSpanAll() ? parent()->flowThreadContainingBlock() : flowThreadContainingBlock();
+    removeFromRenderFlowThreadRecursive(flowThread);
 }
 
 void RenderObject::removeFromRenderFlowThreadRecursive(RenderFlowThread* renderFlowThread)
 {
     if (const RenderObjectChildList* children = virtualChildren()) {
         for (RenderObject* child = children->firstChild(); child; child = child->nextSibling())
             child->removeFromRenderFlowThreadRecursive(renderFlowThread);
     }
 
     if (renderFlowThread && renderFlowThread != this)
         renderFlowThread->flowThreadDescendantWillBeRemoved(this);
     setFlowThreadState(NotInsideFlowThread);
+    RELEASE_ASSERT(!spannerPlaceholder());
 }
 
 void RenderObject::destroyAndCleanupAnonymousWrappers()
 {
     // If the tree is destroyed, there is no need for a clean-up phase.
     if (documentBeingDestroyed()) {
         destroy();
         return;
     }
 
@@ skipping 722 matching lines @@
 {
     if (object1) {
         const blink::RenderObject* root = object1;
         while (root->parent())
             root = root->parent();
         root->showRenderTreeAndMark(object1, "*", object2, "-", 0);
     }
 }
 
 #endif