Have Node::document() return a reference instead of a pointer

Source/core/inspector/InspectorCanvasAgent.cpp

 /*
  * Copyright (C) 2012 Google 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:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 254 matching lines @@
     class NodeVisitor : public WrappedNodeVisitor {
     public:
         NodeVisitor(Page* page, FramesWithUninstrumentedCanvases& result)
             : m_page(page)
             , m_framesWithUninstrumentedCanvases(result)
         {
         }
 
         virtual void visitNode(Node* node) OVERRIDE
         {
-            if (!node->hasTagName(HTMLNames::canvasTag) || !node->document() || !node->document()->frame())
+            if (!node->hasTagName(HTMLNames::canvasTag) || !node->document().frame())
                 return;
 
-            Frame* frame = node->document()->frame();
+            Frame* frame = node->document().frame();
             if (frame->page() != m_page)
                 return;
 
             if (toHTMLCanvasElement(node)->renderingContext())
                 m_framesWithUninstrumentedCanvases.set(frame, true);
         }
 
     private:
         Page* m_page;
         FramesWithUninstrumentedCanvases& m_framesWithUninstrumentedCanvases;
@@ skipping 54 matching lines @@
     ErrorString error;
     for (FramesWithUninstrumentedCanvases::const_iterator it = m_framesWithUninstrumentedCanvases.begin(); it != m_framesWithUninstrumentedCanvases.end(); ++it) {
         InjectedScriptCanvasModule module = injectedScriptCanvasModule(&error, mainWorldScriptState(it->key));
         if (!module.hasNoValue())
             module.markFrameEnd();
     }
 }
 
 } // namespace WebCore