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

Source/core/dom/ContainerNode.cpp

 /*
  * 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 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
  * version 2 of the License, or (at your option) any later version.
@@ skipping 129 matching lines @@
     for (Node* node = child->firstChild(); node; node = node->nextSibling()) {
         if (!newParent->childTypeAllowed(node->nodeType()))
             return false;
     }
     return true;
 }
 
 static inline bool isInTemplateContent(const Node* node)
 {
     Document* document = node->document();
-    return document && document == document->templateDocument();
+    return document == document->templateDocument();
 }
 
 static inline bool containsConsideringHostElements(const Node* newChild, const Node* newParent)
 {
     return (newParent->isInShadowTree() || isInTemplateContent(newParent))
         ? newChild->containsIncludingHostElements(newParent)
         : newChild->contains(newParent);
 }
 
 static inline bool checkAcceptChild(ContainerNode* newParent, Node* newChild, Node* oldChild, ExceptionState& es)
@@ skipping 895 matching lines @@
         return true;
 
     if (node->isElementNode() && toElement(node)->shadow())
         return true;
 
     return false;
 }
 #endif
 
 } // namespace WebCore