Use const ref for LocalFrame::LocalFrameRoot and FrameTree::Top

third_party/WebKit/Source/core/page/FrameTree.h

 /*
  * Copyright (C) 2006 Apple Computer, Inc.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
  * This library is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 25 matching lines @@
   explicit FrameTree(Frame* this_frame);
   ~FrameTree();
 
   const AtomicString& GetName() const;
   void SetName(const AtomicString&);
 
   // TODO(andypaicu): remove this once we have gathered the data
   void ExperimentalSetNulledName();
 
   Frame* Parent() const;
-  Frame* Top() const;
+  Frame& Top() const;
   Frame* NextSibling() const;
   Frame* FirstChild() const;
 
   bool IsDescendantOf(const Frame* ancestor) const;
   Frame* TraverseNext(const Frame* stay_within = nullptr) const;
 
   Frame* Find(const AtomicString& name) const;
   unsigned ChildCount() const;
 
   Frame* ScopedChild(unsigned index) const;
@@ skipping 15 matching lines @@
 };
 
 }  // namespace blink
 
 #ifndef NDEBUG
 // Outside the WebCore namespace for ease of invocation from gdb.
 void showFrameTree(const blink::Frame*);
 #endif
 
 #endif  // FrameTree_h