Have Frame::tree() return a reference

Source/core/rendering/TextAutosizer.cpp

Index: Source/core/rendering/TextAutosizer.cpp
diff --git a/Source/core/rendering/TextAutosizer.cpp b/Source/core/rendering/TextAutosizer.cpp
index 9a2bb5969a85cee9004de3d5bfdb5b7eee182771..f9b8686d41f6fa798fcabf44e3f283537821db21 100644
--- a/Source/core/rendering/TextAutosizer.cpp
+++ b/Source/core/rendering/TextAutosizer.cpp
@@ -187,7 +187,7 @@ bool TextAutosizer::processSubtree(RenderObject* layoutRoot)
     // Largest area of block that can be visible at once (assuming the main
     // frame doesn't get scaled to less than overview scale), in CSS pixels.
     windowInfo.minLayoutSize = mainFrame->view()->layoutSize();
-    for (Frame* frame = m_document->frame(); frame; frame = frame->tree()->parent())
+    for (Frame* frame = m_document->frame(); frame; frame = frame->tree().parent())
         windowInfo.minLayoutSize = windowInfo.minLayoutSize.shrunkTo(frame->view()->layoutSize());
 
     // The layoutRoot could be neither a container nor a cluster, so walk up the tree till we find each of these.