Have Frame::tree() return a reference

Source/core/dom/TreeScope.cpp

Index: Source/core/dom/TreeScope.cpp
diff --git a/Source/core/dom/TreeScope.cpp b/Source/core/dom/TreeScope.cpp
index 9fea0368cb018b072de72b14560be0fefb7645ba..2c5a7489516971834b559537081c70cbf2cacc35 100644
--- a/Source/core/dom/TreeScope.cpp
+++ b/Source/core/dom/TreeScope.cpp
@@ -346,8 +346,8 @@ void TreeScope::adoptIfNeeded(Node* node)
 
 static Element* focusedFrameOwnerElement(Frame* focusedFrame, Frame* currentFrame)
 {
-    for (; focusedFrame; focusedFrame = focusedFrame->tree()->parent()) {
-        if (focusedFrame->tree()->parent() == currentFrame)
+    for (; focusedFrame; focusedFrame = focusedFrame->tree().parent()) {
+        if (focusedFrame->tree().parent() == currentFrame)
             return focusedFrame->ownerElement();
     }
     return 0;