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

Source/core/dom/Position.cpp

 /*
  * Copyright (C) 2004, 2005, 2006, 2009 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 410 matching lines @@
     Position prevPosition = upstream(CanCrossEditingBoundary);
     if (atLastEditingPositionForNode() && prevPosition.isNotNull() && !prevPosition.deprecatedNode()->rendererIsEditable())
         return true;
 
     return nextPosition.isNotNull() && !nextPosition.deprecatedNode()->rendererIsEditable()
         && prevPosition.isNotNull() && !prevPosition.deprecatedNode()->rendererIsEditable();
 }
 
 Node* Position::parentEditingBoundary() const
 {
-    if (!m_anchorNode || !m_anchorNode->document())
+    if (!m_anchorNode)
         return 0;
 
     Node* documentElement = m_anchorNode->document()->documentElement();
     if (!documentElement)
         return 0;
 
     Node* boundary = m_anchorNode.get();
     while (boundary != documentElement && boundary->nonShadowBoundaryParentNode() && m_anchorNode->rendererIsEditable() == boundary->parentNode()->rendererIsEditable())
         boundary = boundary->nonShadowBoundaryParentNode();
 
@@ skipping 948 matching lines @@
     pos.showTreeForThis();
 }
 
 void showTree(const WebCore::Position* pos)
 {
     if (pos)
         pos->showTreeForThis();
 }
 
 #endif