Have Frame::tree() return a reference

Source/core/dom/FullscreenElementStack.cpp

Index: Source/core/dom/FullscreenElementStack.cpp
diff --git a/Source/core/dom/FullscreenElementStack.cpp b/Source/core/dom/FullscreenElementStack.cpp
index 550bd427c64f8c2e46b967b9d36948569bc3f5fe..1bfc607bef4e8e70a4bffb0fe90c851a4fecd679 100644
--- a/Source/core/dom/FullscreenElementStack.cpp
+++ b/Source/core/dom/FullscreenElementStack.cpp
@@ -171,7 +171,7 @@ void FullscreenElementStack::requestFullScreenForElement(Element* element, unsig
 
         // A descendant browsing context's document has a non-empty fullscreen element stack.
         bool descendentHasNonEmptyStack = false;
-        for (Frame* descendant = document()->frame() ? document()->frame()->tree()->traverseNext() : 0; descendant; descendant = descendant->tree()->traverseNext()) {
+        for (Frame* descendant = document()->frame() ? document()->frame()->tree().traverseNext() : 0; descendant; descendant = descendant->tree().traverseNext()) {
             if (fullscreenElementFrom(descendant->document())) {
                 descendentHasNonEmptyStack = true;
                 break;
@@ -285,7 +285,7 @@ void FullscreenElementStack::webkitExitFullscreen()
     // element stack (if any), ordered so that the child of the doc is last and the document furthest
     // away from the doc is first.
     Deque<RefPtr<Document> > descendants;
-    for (Frame* descendant = document()->frame() ?  document()->frame()->tree()->traverseNext() : 0; descendant; descendant = descendant->tree()->traverseNext()) {
+    for (Frame* descendant = document()->frame() ?  document()->frame()->tree().traverseNext() : 0; descendant; descendant = descendant->tree().traverseNext()) {
         if (fullscreenElementFrom(descendant->document()))
             descendants.prepend(descendant->document());
     }