Have Frame::tree() return a reference

Source/core/editing/FrameSelection.cpp

Index: Source/core/editing/FrameSelection.cpp
diff --git a/Source/core/editing/FrameSelection.cpp b/Source/core/editing/FrameSelection.cpp
index 4fb8a6ecb7f07e380079ab73f5a013994d89816c..4531c09abb7b5c22281f9b8e8387df7f76edb1c2 100644
--- a/Source/core/editing/FrameSelection.cpp
+++ b/Source/core/editing/FrameSelection.cpp
@@ -1342,7 +1342,7 @@ bool FrameSelection::contains(const LayoutPoint& point)
 void FrameSelection::selectFrameElementInParentIfFullySelected()
 {
     // Find the parent frame; if there is none, then we have nothing to do.
-    Frame* parent = m_frame->tree()->parent();
+    Frame* parent = m_frame->tree().parent();
     if (!parent)
         return;
     Page* page = m_frame->page();