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

third_party/WebKit/Source/core/dom/DOMImplementation.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008 Apple Inc. All rights reserved.
  * Copyright (C) 2006 Samuel Weinig (sam@webkit.org)
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved.
  * (http://www.torchmobile.com/)
  *
  * This library is free software; you can redistribute it and/or
@@ skipping 230 matching lines @@
     // init.frame()->tree().top()->securityContext() returns nullptr.
     // For that reason, the origin must be retrieved directly from init.url().
     if (init.GetFrame()->IsMainFrame()) {
       RefPtr<SecurityOrigin> origin = SecurityOrigin::Create(init.Url());
       plugin_data = init.GetFrame()->GetPage()->GetPluginData(origin.Get());
     } else {
       plugin_data =
           init.GetFrame()->GetPage()->GetPluginData(init.GetFrame()
                                                         ->Tree()
                                                         .Top()
-                                                        ->GetSecurityContext()
+                                                        .GetSecurityContext()
                                                         ->GetSecurityOrigin());
     }
   }
 
   // PDF is one image type for which a plugin can override built-in support.
   // We do not want QuickTime to take over all image types, obviously.
   if ((type == "application/pdf" || type == "text/pdf") && plugin_data &&
       plugin_data->SupportsMimeType(type))
     return PluginDocument::Create(init);
   // multipart/x-mixed-replace is only supported for images.
@@ skipping 21 matching lines @@
     return XMLDocument::Create(init);
 
   return HTMLDocument::Create(init);
 }
 
 DEFINE_TRACE(DOMImplementation) {
   visitor->Trace(document_);
 }
 
 }  // namespace blink