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

third_party/WebKit/Source/core/frame/LocalDOMWindow.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies)
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 1619 matching lines @@
     // for empty frame names.  Otherwise, illegitimate window.open() calls with
     // no name will pass right through the popup blocker.
     if (frame_name.IsEmpty() || !GetFrame()->Tree().Find(frame_name))
       return nullptr;
   }
 
   // Get the target frame for the special cases of _top and _parent.
   // In those cases, we schedule a location change right now and return early.
   Frame* target_frame = nullptr;
   if (EqualIgnoringASCIICase(frame_name, "_top")) {
-    target_frame = GetFrame()->Tree().Top();
+    target_frame = &GetFrame()->Tree().Top();
   } else if (EqualIgnoringASCIICase(frame_name, "_parent")) {
     if (Frame* parent = GetFrame()->Tree().Parent())
       target_frame = parent;
     else
       target_frame = GetFrame();
   }
 
   if (target_frame) {
     if (!active_document->GetFrame() ||
         !active_document->GetFrame()->CanNavigate(*target_frame))
@@ skipping 42 matching lines @@
   DOMWindow::Trace(visitor);
   Supplementable<LocalDOMWindow>::Trace(visitor);
 }
 
 DEFINE_TRACE_WRAPPERS(LocalDOMWindow) {
   visitor->TraceWrappers(custom_elements_);
   DOMWindow::TraceWrappers(visitor);
 }
 
 }  // namespace blink