Have Frame::tree() return a reference

Source/core/page/SpatialNavigation.cpp

Index: Source/core/page/SpatialNavigation.cpp
diff --git a/Source/core/page/SpatialNavigation.cpp b/Source/core/page/SpatialNavigation.cpp
index 57ac410a8678d21c1370b330dc3aa0429e0c72d8..e75aabd2755edd18b161d4a6ce745c40fbaad39c 100644
--- a/Source/core/page/SpatialNavigation.cpp
+++ b/Source/core/page/SpatialNavigation.cpp
@@ -502,7 +502,7 @@ bool canScrollInDirection(const Frame* frame, FocusDirection direction)
 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());