Add a use counter for async XHR.

Source/core/xmlhttprequest/XMLHttpRequest.cpp

 /*
  *  Copyright (C) 2004, 2006, 2008 Apple Inc. All rights reserved.
  *  Copyright (C) 2005-2007 Alexey Proskuryakov <ap@webkit.org>
  *  Copyright (C) 2007, 2008 Julien Chaffraix <jchaffraix@webkit.org>
  *  Copyright (C) 2008, 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
@@ skipping 982 matching lines @@
 
     if (responseTypeCode() == ResponseTypeStream) {
         request.setUseStreamOnResponse(true);
         resourceLoaderOptions.dataBufferingPolicy = DoNotBufferData;
     }
 
     m_exceptionCode = 0;
     m_error = false;
 
     if (m_async) {
+        UseCounter::count(&executionContext, UseCounter::XMLHttpRequestAsynchronous);

[tyoshino (SeeGerritForStatus), 2015/01/27 05:07:42]

This counts sendForInspectorXHRReplay(), too. It's for DevTools. But maybe it's
ok as on that page there the XHR has already happened and was counted, so
sendForInspectorXHRReplay() would be no-op regarding UseCounter::count().

Also we already have one for sync XHR at L1015. Let's just align with it.

         if (m_upload)
             request.setReportUploadProgress(true);
 
         // ThreadableLoader::create can return null here, for example if we're no longer attached to a page.
         // This is true while running onunload handlers.
         // FIXME: Maybe we need to be able to send XMLHttpRequests from onunload, <http://bugs.webkit.org/show_bug.cgi?id=10904>.
         // FIXME: Maybe create() can return null for other reasons too?
         ASSERT(!m_loader);
         m_loader = ThreadableLoader::create(executionContext, this, request, options, resourceLoaderOptions);
     } else {
@@ skipping 790 matching lines @@
     visitor->trace(m_responseDocumentParser);
     visitor->trace(m_progressEventThrottle);
     visitor->trace(m_upload);
     visitor->trace(m_blobLoader);
     XMLHttpRequestEventTarget::trace(visitor);
     DocumentParserClient::trace(visitor);
     ActiveDOMObject::trace(visitor);
 }
 
 } // namespace blink