Have Frame::tree() return a reference

Source/core/html/HTMLDocument.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  * Copyright (C) 2003, 2004, 2005, 2006, 2007, 2008 Apple Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
@@ skipping 106 matching lines @@
 }
 
 bool HTMLDocument::hasFocus()
 {
     Page* page = this->page();
     if (!page)
         return false;
     if (!page->focusController().isActive() || !page->focusController().isFocused())
         return false;
     if (Frame* focusedFrame = page->focusController().focusedFrame()) {
-        if (focusedFrame->tree()->isDescendantOf(frame()))
+        if (focusedFrame->tree().isDescendantOf(frame()))
             return true;
     }
     return false;
 }
 
 HTMLBodyElement* HTMLDocument::htmlBodyElement() const
 {
     HTMLElement* body = this->body();
     return (body && body->hasTagName(bodyTag)) ? toHTMLBodyElement(body) : 0;
 }
@@ skipping 182 matching lines @@
 }
 
 void HTMLDocument::clear()
 {
     // FIXME: This does nothing, and that seems unlikely to be correct.
     // We've long had a comment saying that IE doesn't support this.
     // But I do see it in the documentation for Mozilla.
 }
 
 }