Have Frame::tree() return a reference

Source/core/page/SpatialNavigation.cpp

 /*
  * Copyright (C) 2009 Nokia Corporation and/or its subsidiary(-ies)
  * Copyright (C) 2009 Antonio Gomes <tonikitoo@webkit.org>
  *
  * 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
@@ skipping 484 matching lines @@
         return rect.height() + offset.height() < size.height();
     default:
         ASSERT_NOT_REACHED();
         return false;
     }
 }
 
 static LayoutRect rectToAbsoluteCoordinates(Frame* initialFrame, const LayoutRect& initialRect)
 {
     LayoutRect rect = initialRect;
-    for (Frame* frame = initialFrame; frame; frame = frame->tree()->parent()) {
+    for (Frame* frame = initialFrame; frame; frame = frame->tree().parent()) {
         if (Element* element = frame->ownerElement()) {
             do {
                 rect.move(element->offsetLeft(), element->offsetTop());
             } while ((element = element->offsetParent()));
             rect.move((-frame->view()->scrollOffset()));
         }
     }
     return rect;
 }
 
@@ skipping 214 matching lines @@
     LayoutRect rect = virtualRectForDirection(direction, rectToAbsoluteCoordinates(area->document().frame(), area->computeRect(area->imageElement()->renderer())), 1);
     return rect;
 }
 
 HTMLFrameOwnerElement* frameOwnerElement(FocusCandidate& candidate)
 {
     return candidate.isFrameOwnerElement() ? toHTMLFrameOwnerElement(candidate.visibleNode) : 0;
 };
 
 } // namespace WebCore