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

Source/web/FullscreenController.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 43 matching lines @@
     , m_isCancelingFullScreen(false)
 {
 }
 
 void FullscreenController::willEnterFullScreen()
 {
     if (!m_provisionalFullScreenElement)
         return;
 
     // Ensure that this element's document is still attached.
-    Document* doc = m_provisionalFullScreenElement->document();
-    if (doc->frame()) {
-        FullscreenElementStack::from(doc)->webkitWillEnterFullScreenForElement(m_provisionalFullScreenElement.get());
-        m_fullScreenFrame = doc->frame();
+    Document& doc = m_provisionalFullScreenElement->document();
+    if (doc.frame()) {
+        FullscreenElementStack::from(&doc)->webkitWillEnterFullScreenForElement(m_provisionalFullScreenElement.get());
+        m_fullScreenFrame = doc.frame();
     }
     m_provisionalFullScreenElement.clear();
 }
 
 void FullscreenController::didEnterFullScreen()
 {
     if (!m_fullScreenFrame)
         return;
 
     if (Document* doc = m_fullScreenFrame->document()) {
@@ skipping 98 matching lines @@
             mediaElement->player()->exitFullscreen();
         return;
     }
 #endif
     if (WebViewClient* client = m_webViewImpl->client())
         client->exitFullScreen();
 }
 
 }