Have Frame::tree() return a reference

Source/core/inspector/InspectorCanvasAgent.cpp

 /*
  * Copyright (C) 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 307 matching lines @@
             it->value = false;
         m_frontend->traceLogsRemoved(0, 0);
     } else {
         while (frame) {
             if (m_framesWithUninstrumentedCanvases.contains(frame))
                 m_framesWithUninstrumentedCanvases.set(frame, false);
             if (m_pageAgent->hasIdForFrame(frame)) {
                 String frameId = m_pageAgent->frameId(frame);
                 m_frontend->traceLogsRemoved(&frameId, 0);
             }
-            frame = frame->tree()->traverseNext();
+            frame = frame->tree().traverseNext();
         }
     }
 }
 
 void InspectorCanvasAgent::frameDetachedFromParent(Frame* frame)
 {
     if (m_enabled)
         m_framesWithUninstrumentedCanvases.remove(frame);
 }
 
 void InspectorCanvasAgent::didBeginFrame()
 {
     if (!m_enabled)
         return;
     ErrorString error;
     for (FramesWithUninstrumentedCanvases::const_iterator it = m_framesWithUninstrumentedCanvases.begin(); it != m_framesWithUninstrumentedCanvases.end(); ++it) {
         InjectedScriptCanvasModule module = injectedScriptCanvasModule(&error, mainWorldScriptState(it->key));
         if (!module.hasNoValue())
             module.markFrameEnd();
     }
 }
 
 } // namespace WebCore