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

Source/core/html/ime/InputMethodContext.cpp

 /*
  * Copyright (C) 2013 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 55 matching lines @@
     return emptyString();
 }
 
 HTMLElement* InputMethodContext::target() const
 {
     return m_element;
 }
 
 void InputMethodContext::confirmComposition()
 {
-    Frame* frame = m_element->document()->frame();
+    Frame* frame = m_element->document().frame();
     if (!frame)
         return;
 
     const Element* element = frame->document()->focusedElement();
     if (!element || !element->isHTMLElement() || m_element != toHTMLElement(element))
         return;
 
     frame->inputMethodController().confirmCompositionAndResetState();
 }
 
 void InputMethodContext::setCaretRectangle(Node* anchor, int x, int y, int w, int h)
 {
     // FIXME: Implement this.
 }
 
 void InputMethodContext::setExclusionRectangle(Node* anchor, int x, int y, int w, int h)
 {
     // FIXME: Implement this.
 }
 
 } // namespace WebCore