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

Source/core/rendering/RenderObject.h

 /*
  * Copyright (C) 2000 Lars Knoll (knoll@kde.org)
  *           (C) 2000 Antti Koivisto (koivisto@kde.org)
  *           (C) 2000 Dirk Mueller (mueller@kde.org)
  *           (C) 2004 Allan Sandfeld Jensen (kde@carewolf.com)
  * Copyright (C) 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2009 Google 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
@@ skipping 581 matching lines @@
     Node* nonPseudoNode() const { return isPseudoElement() ? 0 : node(); }
 
     // FIXME: Why does RenderWidget need this?
     void clearNode() { m_node = 0; }
 
     // Returns the styled node that caused the generation of this renderer.
     // This is the same as node() except for renderers of :before and :after
     // pseudo elements for which their parent node is returned.
     Node* generatingNode() const { return isPseudoElement() ? node()->parentOrShadowHostNode() : node(); }
 
+    // FIXME: This should return a reference.
     Document* document() const { return m_node->document(); }
     Frame* frame() const { return document()->frame(); }
 
     bool hasOutlineAnnotation() const;
     bool hasOutline() const { return style()->hasOutline() || hasOutlineAnnotation(); }
 
     // Returns the object containing this one. Can be different from parent for positioned elements.
     // If repaintContainer and repaintContainerSkipped are not null, on return *repaintContainerSkipped
     // is true if the renderer returned is an ancestor of repaintContainer.
     RenderObject* container(const RenderLayerModelObject* repaintContainer = 0, bool* repaintContainerSkipped = 0) const;
@@ skipping 778 matching lines @@
 // Outside the WebCore namespace for ease of invocation from gdb.
 void showTree(const WebCore::RenderObject*);
 void showLineTree(const WebCore::RenderObject*);
 void showRenderTree(const WebCore::RenderObject* object1);
 // We don't make object2 an optional parameter so that showRenderTree
 // can be called from gdb easily.
 void showRenderTree(const WebCore::RenderObject* object1, const WebCore::RenderObject* object2);
 #endif
 
 #endif // RenderObject_h