Have Frame::tree() return a reference

Source/core/page/PageGroup.cpp

 /*
  * Copyright (C) 2008 Apple 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:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 67 matching lines @@
 {
     m_injectedStyleSheets.clear();
     invalidatedInjectedStyleSheetCacheInAllFrames();
 }
 
 void PageGroup::invalidatedInjectedStyleSheetCacheInAllFrames()
 {
     // Clear our cached sheets and have them just reparse.
     HashSet<Page*>::const_iterator end = m_pages.end();
     for (HashSet<Page*>::const_iterator it = m_pages.begin(); it != end; ++it) {
-        for (Frame* frame = (*it)->mainFrame(); frame; frame = frame->tree()->traverseNext())
+        for (Frame* frame = (*it)->mainFrame(); frame; frame = frame->tree().traverseNext())
             frame->document()->styleEngine()->invalidateInjectedStyleSheetCache();
     }
 }
 
 } // namespace WebCore