Have Frame::tree() return a reference

Source/core/page/PageSerializer.cpp

 /*
  * Copyright (C) 2011 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 230 matching lines @@
                 serializeCSSStyleSheet(sheet, url);
                 ASSERT(m_resourceURLs.contains(url));
             }
         } else if (element->hasTagName(HTMLNames::styleTag)) {
             HTMLStyleElement* styleElement = toHTMLStyleElement(element);
             if (CSSStyleSheet* sheet = styleElement->sheet())
                 serializeCSSStyleSheet(sheet, KURL());
         }
     }
 
-    for (Frame* childFrame = frame->tree()->firstChild(); childFrame; childFrame = childFrame->tree()->nextSibling())
+    for (Frame* childFrame = frame->tree().firstChild(); childFrame; childFrame = childFrame->tree().nextSibling())
         serializeFrame(childFrame);
 }
 
 void PageSerializer::serializeCSSStyleSheet(CSSStyleSheet* styleSheet, const KURL& url)
 {
     StringBuilder cssText;
     for (unsigned i = 0; i < styleSheet->length(); ++i) {
         CSSRule* rule = styleSheet->item(i);
         String itemText = rule->cssText();
         if (!itemText.isEmpty()) {
@@ skipping 114 matching lines @@
     if (iter != m_blankFrameURLs.end())
         return iter->value;
     String url = "wyciwyg://frame/" + String::number(m_blankFrameCounter++);
     KURL fakeURL(ParsedURLString, url);
     m_blankFrameURLs.add(frame, fakeURL);
 
     return fakeURL;
 }
 
 }