Have Frame::tree() return a reference

Source/core/inspector/InspectorFileSystemAgent.cpp

 /*
  * Copyright (C) 2011, 2012 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 684 matching lines @@
 {
     if (!m_enabled) {
         *error = "FileSystem agent is not enabled.";
         return false;
     }
     return true;
 }
 
 ExecutionContext* InspectorFileSystemAgent::assertExecutionContextForOrigin(ErrorString* error, SecurityOrigin* origin)
 {
-    for (Frame* frame = m_pageAgent->mainFrame(); frame; frame = frame->tree()->traverseNext()) {
+    for (Frame* frame = m_pageAgent->mainFrame(); frame; frame = frame->tree().traverseNext()) {
         if (frame->document() && frame->document()->securityOrigin()->isSameSchemeHostPort(origin))
             return frame->document();
     }
 
     *error = "No frame is available for the request";
     return 0;
 }
 
 } // namespace WebCore