Have Frame::tree() return a reference

Source/core/dom/TouchController.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 copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 79 matching lines @@
         return;
     }
 
     Page* page = document->page();
     if (!page)
         return;
     if (ScrollingCoordinator* scrollingCoordinator = page->scrollingCoordinator())
         scrollingCoordinator->touchEventTargetRectsDidChange(document);
     if (m_touchEventTargets->size())
         return;
-    for (const Frame* frame = page->mainFrame(); frame; frame = frame->tree()->traverseNext()) {
+    for (const Frame* frame = page->mainFrame(); frame; frame = frame->tree().traverseNext()) {
         if (frame->document() && TouchController::from(frame->document())->hasTouchEventHandlers())
             return;
     }
     page->chrome().client().needTouchEvents(false);
 }
 
 void TouchController::didRemoveEventTargetNode(Document* document, Node* handler)
 {
     if (m_touchEventTargets && !m_touchEventTargets->isEmpty()) {
         if (handler == document)
@@ skipping 42 matching lines @@
 
 void TouchController::documentBeingDestroyed()
 {
     Document* document = static_cast<Document*>(executionContext());
 
     if (Document* ownerDocument = document->ownerDocument())
         TouchController::from(ownerDocument)->didRemoveEventTargetNode(ownerDocument, document);
 }
 
 } // namespace WebCore