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

Source/core/dom/NodeIterator.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  * Copyright (C) 2000 Frederik Holljen (frederik.holljen@hig.no)
  * Copyright (C) 2001 Peter Kelly (pmk@post.com)
  * Copyright (C) 2006 Samuel Weinig (sam.weinig@gmail.com)
  * Copyright (C) 2004, 2008 Apple 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
  * License as published by the Free Software Foundation; either
@@ skipping 60 matching lines @@
     }
     node = NodeTraversal::previous(node.get(), root);
     return node;
 }
 
 NodeIterator::NodeIterator(PassRefPtr<Node> rootNode, unsigned whatToShow, PassRefPtr<NodeFilter> filter)
     : Traversal(rootNode, whatToShow, filter)
     , m_referenceNode(root(), true)
     , m_detached(false)
 {
-    // Document type nodes may have a null document. But since they can't have children, there is no need to listen for modifications to these.
-    ASSERT(root()->document() || root()->nodeType() == Node::DOCUMENT_TYPE_NODE);
     ScriptWrappable::init(this);
-    if (Document* ownerDocument = root()->document())
-        ownerDocument->attachNodeIterator(this);
+    root()->document()->attachNodeIterator(this);
 }
 
 NodeIterator::~NodeIterator()
 {
-    if (Document* ownerDocument = root()->document())
-        ownerDocument->detachNodeIterator(this);
+    root()->document()->detachNodeIterator(this);
 }
 
 PassRefPtr<Node> NodeIterator::nextNode(ScriptState* state, ExceptionState& es)
 {
     if (m_detached) {
         es.throwDOMException(InvalidStateError);
         return 0;
     }
 
     RefPtr<Node> result;
@@ skipping 42 matching lines @@
             break;
         }
     }
 
     m_candidateNode.clear();
     return result.release();
 }
 
 void NodeIterator::detach()
 {
-    if (Document* ownerDocument = root()->document())
-        ownerDocument->detachNodeIterator(this);
+    root()->document()->detachNodeIterator(this);
     m_detached = true;
     m_referenceNode.node.clear();
 }
 
 void NodeIterator::nodeWillBeRemoved(Node* removedNode)
 {
     updateForNodeRemoval(removedNode, m_candidateNode);
     updateForNodeRemoval(removedNode, m_referenceNode);
 }
 
@@ skipping 61 matching lines @@
                     node = NodeTraversal::previous(node, root());
             }
             if (node)
                 referenceNode.node = node;
         }
     }
 }
 
 
 } // namespace WebCore