Enable ancient assert in LayoutTreeBuilder.

Source/core/dom/LayoutTreeBuilder.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  * Copyright (C) 2011 Google Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
@@ skipping 30 matching lines @@
 class LayoutTreeBuilder {
     STACK_ALLOCATED();
 protected:
     LayoutTreeBuilder(NodeType& node, LayoutObject* layoutObjectParent)
         : m_node(node)
         , m_layoutObjectParent(layoutObjectParent)
     {
         ASSERT(!node.layoutObject());
         ASSERT(node.needsAttach());
         ASSERT(node.document().inStyleRecalc());
-
-        // FIXME: We should be able to ASSERT(node->inActiveDocument()) but childrenChanged is called
-        // before ChildNodeInsertionNotifier in ContainerNode's methods and some implementations
-        // will trigger a layout inside childrenChanged.
-        // Mainly HTMLTextAreaElement::childrenChanged calls HTMLTextFormControlElement::setSelectionRange
-        // which does an updateLayoutIgnorePendingStylesheets.
+        ASSERT(node.inActiveDocument());
     }
 
     LayoutObject* nextLayoutObject() const
     {
         ASSERT(m_layoutObjectParent);
 
         // Avoid an O(N^2) walk over the children when reattaching all children of a node.
         if (m_layoutObjectParent->node() && m_layoutObjectParent->node()->needsAttach())
             return 0;
 
@@ skipping 28 matching lines @@
 public:
     LayoutTreeBuilderForText(Text& text, LayoutObject* layoutParent)
         : LayoutTreeBuilder(text, layoutParent) { }
 
     void createLayoutObject();
 };
 
 } // namespace blink
 
 #endif