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

Source/core/page/Chrome.cpp

 /*
  * Copyright (C) 2006, 2007, 2009, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2008, 2010 Nokia Corporation and/or its subsidiary(-ies)
  * Copyright (C) 2012, Samsung Electronics. 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
  * version 2 of the License, or (at your option) any later version.
  *
@@ skipping 288 matching lines @@
 }
 
 IntRect Chrome::windowResizerRect() const
 {
     return m_client->windowResizerRect();
 }
 
 void Chrome::mouseDidMoveOverElement(const HitTestResult& result, unsigned modifierFlags)
 {
     if (result.innerNode()) {
-        Document* document = result.innerNode()->document();
-        if (document && document->isDNSPrefetchEnabled())
+        if (result.innerNode()->document()->isDNSPrefetchEnabled())
             prefetchDNS(result.absoluteLinkURL().host());
     }
     m_client->mouseDidMoveOverElement(result, modifierFlags);
 }
 
 void Chrome::setToolTip(const HitTestResult& result)
 {
     // First priority is a potential toolTip representing a spelling or grammar error
     TextDirection toolTipDirection;
     String toolTip = result.spellingToolTip(toolTipDirection);
@@ skipping 93 matching lines @@
 }
 
 void Chrome::notifyPopupOpeningObservers() const
 {
     const Vector<PopupOpeningObserver*> observers(m_popupOpeningObservers);
     for (size_t i = 0; i < observers.size(); ++i)
         observers[i]->willOpenPopup();
 }
 
 } // namespace WebCore