Remove useless null checks from Node::document() call sites

Source/core/dom/Node.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/)
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
@@ skipping 463 matching lines @@
     IntRect pixelSnappedRenderRect(bool* isReplaced) { return pixelSnappedIntRect(renderRect(isReplaced)); }
 
     // Returns true if the node has a non-empty bounding box in layout.
     // This does not 100% guarantee the user can see it, but is pretty close.
     // Note: This method only works properly after layout has occurred.
     bool hasNonEmptyBoundingBox() const;
 
     unsigned nodeIndex() const;
 
     // Returns the DOM ownerDocument attribute. This method never returns NULL, except in the case
-    // of (1) a Document node or (2) a DocumentType node that is not used with any Document yet.
+    // of a Document node.
     Document* ownerDocument() const;
 
-    // Returns the document associated with this node. This method never returns NULL, except in the case
-    // of a DocumentType node that is not used with any Document yet. A Document node returns itself.
+    // FIXME: This should return a reference.
+    // Returns the document associated with this node. This method never returns 0.
+    // A Document node returns itself.
     Document* document() const
     {
         ASSERT(this);
-        // FIXME: below ASSERT is useful, but prevents the use of document() in the constructor or destructor
-        // due to the virtual function call to nodeType().
-        ASSERT(documentInternal() || (nodeType() == DOCUMENT_TYPE_NODE && !inDocument()));
+        ASSERT(documentInternal());
         return documentInternal();
     }
 
     TreeScope* treeScope() const { return m_treeScope; }
 
     // Returns true if this node is associated with a document and is in its associated document's
     // node tree, false otherwise.
     bool inDocument() const
     {
         ASSERT(documentInternal() || !getFlag(InDocumentFlag));
@@ skipping 439 matching lines @@
 
 } //namespace
 
 #ifndef NDEBUG
 // Outside the WebCore namespace for ease of invocation from gdb.
 void showTree(const WebCore::Node*);
 void showNodePath(const WebCore::Node*);
 #endif
 
 #endif