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

Source/core/html/shadow/SliderThumbElement.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2009 Apple Inc. All rights reserved.
  * Copyright (C) 2010 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.
@@ skipping 294 matching lines @@
 
     // FIXME: This is no longer being set from renderer. Consider updating the method name.
     input->setValueFromRenderer(valueString);
     if (renderer())
         renderer()->setNeedsLayout();
     input->dispatchFormControlChangeEvent();
 }
 
 void SliderThumbElement::startDragging()
 {
-    if (Frame* frame = document()->frame()) {
+    if (Frame* frame = document().frame()) {
         frame->eventHandler()->setCapturingMouseEventsNode(this);
         m_inDragMode = true;
     }
 }
 
 void SliderThumbElement::stopDragging()
 {
     if (!m_inDragMode)
         return;
 
-    if (Frame* frame = document()->frame())
+    if (Frame* frame = document().frame())
         frame->eventHandler()->setCapturingMouseEventsNode(0);
     m_inDragMode = false;
     if (renderer())
         renderer()->setNeedsLayout();
 }
 
 void SliderThumbElement::defaultEventHandler(Event* event)
 {
     if (!event->isMouseEvent()) {
         HTMLDivElement::defaultEventHandler(event);
@@ skipping 45 matching lines @@
     const HTMLInputElement* input = hostInput();
     if (input && !input->isDisabledOrReadOnly())
         return true;
 
     return HTMLDivElement::willRespondToMouseClickEvents();
 }
 
 void SliderThumbElement::detach(const AttachContext& context)
 {
     if (m_inDragMode) {
-        if (Frame* frame = document()->frame())
+        if (Frame* frame = document().frame())
             frame->eventHandler()->setCapturingMouseEventsNode(0);
     }
     HTMLDivElement::detach(context);
 }
 
 HTMLInputElement* SliderThumbElement::hostInput() const
 {
     // Only HTMLInputElement creates SliderThumbElement instances as its shadow nodes.
     // So, shadowHost() must be an HTMLInputElement.
     return toHTMLInputElement(shadowHost());
@@ skipping 64 matching lines @@
     case MediaVolumeSliderThumbPart:
     case MediaFullScreenVolumeSliderPart:
     case MediaFullScreenVolumeSliderThumbPart:
         return mediaSliderContainer;
     default:
         return sliderContainer;
     }
 }
 
 }