Show a warning when using sync xhr.

Source/core/dom/Document.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  *           (C) 2006 Alexey Proskuryakov (ap@webkit.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  * Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies)
  * Copyright (C) 2011 Google Inc. All rights reserved.
  *
@@ skipping 887 matching lines @@
         BeforeUnloadEventInProgress,
         BeforeUnloadEventCompleted,
         PageHideInProgress,
         UnloadEventInProgress,
         UnloadEventHandled
     };
     bool loadEventStillNeeded() const { return m_loadEventProgress == LoadEventNotRun; }
     bool processingLoadEvent() const { return m_loadEventProgress == LoadEventInProgress; }
     bool loadEventFinished() const { return m_loadEventProgress >= LoadEventCompleted; }
     bool unloadStarted() const { return m_loadEventProgress >= PageHideInProgress; }
+    bool processingBeforeUnload() const { return m_loadEventProgress == BeforeUnloadEventInProgress; }
 
     void setContainsPlugins() { m_containsPlugins = true; }
     bool containsPlugins() const { return m_containsPlugins; }
 
     virtual bool isContextThread() const override final;
     virtual bool isJSExecutionForbidden() const override final { return false; }
 
     bool containsValidityStyleRules() const { return m_containsValidityStyleRules; }
     void setContainsValidityStyleRules() { m_containsValidityStyleRules = true; }
 
@@ skipping 500 matching lines @@
 Node* eventTargetNodeForDocument(Document*);
 
 } // namespace blink
 
 #ifndef NDEBUG
 // Outside the WebCore namespace for ease of invocation from gdb.
 void showLiveDocumentInstances();
 #endif
 
 #endif // Document_h