[DevTools] Log failed XHR requests.

Source/core/xmlhttprequest/XMLHttpRequest.h

 /*
  *  Copyright (C) 2003, 2006, 2008 Apple Inc. All rights reserved.
  *  Copyright (C) 2005, 2006 Alexey Proskuryakov <ap@nypop.com>
  *  Copyright (C) 2011 Google Inc. All rights reserved.
  *  Copyright (C) 2012 Intel Corporation
  *
  *  This library is free software; you can redistribute it and/or
  *  modify it under the terms of the GNU Lesser General Public
  *  License as published by the Free Software Foundation; either
  *  version 2 of the License, or (at your option) any later version.
@@ skipping 251 matching lines @@
     // Not converted to ASCII lowercase. Must be lowered later or compared
     // using case insensitive comparison functions if needed.
     AtomicString m_mimeTypeOverride;
     unsigned long m_timeoutMilliseconds;
     PersistentWillBeMember<Blob> m_responseBlob;
     PersistentWillBeMember<Stream> m_responseLegacyStream;
     PersistentWillBeMember<ReadableStreamImpl<ReadableStreamChunkTypeTraits<DOMArrayBufferView>>> m_responseStream;
     PersistentWillBeMember<ReadableStreamSource> m_responseStreamSource;
 
     RefPtr<ThreadableLoader> m_loader;
-    unsigned long m_loaderIdentifier;
     State m_state;
 
     ResourceResponse m_response;
     String m_finalResponseCharset;
 
     OwnPtr<TextResourceDecoder> m_decoder;
 
     ScriptString m_responseText;
     RefPtrWillBeMember<Document> m_responseDocument;
     RefPtrWillBeMember<DocumentParser> m_responseDocumentParser;
@@ skipping 37 matching lines @@
     bool m_sameOriginRequest;
     // True iff the ongoing resource loading is using the downloadToFile
     // option.
     bool m_downloadingToFile;
     bool m_responseTextOverflow;
 };
 
 } // namespace blink
 
 #endif // XMLHttpRequest_h