Keep track of multiple layout roots

Source/core/layout/LayoutObject.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 2256 matching lines @@
 }
 
 bool LayoutObject::isSelectionBorder() const
 {
     SelectionState st = selectionState();
     return st == SelectionStart || st == SelectionEnd || st == SelectionBoth;
 }
 
 inline void LayoutObject::clearLayoutRootIfNeeded() const
 {
-    if (frame()) {
-        if (FrameView* view = frame()->view()) {
-            if (view->layoutRoot() == this) {
-                if (!documentBeingDestroyed())
-                    ASSERT_NOT_REACHED();
-                // This indicates a failure to layout the child, which is why
-                // the layout root is still set to |this|. Make sure to clear it
-                // since we are getting destroyed.
-                view->clearLayoutSubtreeRoot();
-            }
-        }
+    if (FrameView* view = frameView()) {
+        if (!documentBeingDestroyed())
+            view->clearLayoutSubtreeRoot(this);
     }
 }
 
 void LayoutObject::willBeDestroyed()
 {
     // Destroy any leftover anonymous children.
     LayoutObjectChildList* children = virtualChildren();
     if (children)
         children->destroyLeftoverChildren();
 
@@ skipping 886 matching lines @@
 {
     if (object1) {
         const blink::LayoutObject* root = object1;
         while (root->parent())
             root = root->parent();
         root->showRenderTreeAndMark(object1, "*", object2, "-", 0);
     }
 }
 
 #endif