Show a warning when using sync 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 589 matching lines @@
         if (m_responseTypeCode != ResponseTypeDefault) {
             exceptionState.throwDOMException(InvalidAccessError, "Synchronous requests from a document must not set a response type.");
             return;
         }
 
         // Similarly, timeouts are disabled for synchronous requests as well.
         if (m_timeoutMilliseconds > 0) {
             exceptionState.throwDOMException(InvalidAccessError, "Synchronous requests must not set a timeout.");
             return;
         }
+
+        // Here we just warn that firing sync XHR's may affect responsiveness.
+        // Eventually sync xhr will be deprecated and an "InvalidAccessError" exception thrown.
+        // Refer : https://xhr.spec.whatwg.org/#sync-warning
+        // Use count for XHR synchronous requests on main thread only.
+        if (!document()->processingBeforeUnload())
+            UseCounter::countDeprecation(executionContext(), UseCounter::XMLHttpRequestSynchronousInNonWorkerOutsideBeforeUnload);
     }
 
     m_method = FetchUtils::normalizeMethod(method);
 
     m_url = url;
 
     m_async = async;
 
     ASSERT(!m_loader);
 
@@ skipping 1035 matching lines @@
     visitor->trace(m_responseDocument);
     visitor->trace(m_responseDocumentParser);
     visitor->trace(m_progressEventThrottle);
     visitor->trace(m_upload);
     visitor->trace(m_blobLoader);
     XMLHttpRequestEventTarget::trace(visitor);
     DocumentParserClient::trace(visitor);
 }
 
 } // namespace blink