Show a warning when using sync xhr.

Source/core/dom/ExecutionContext.h

 /*
  * Copyright (C) 2008 Apple Inc. All Rights Reserved.
  * 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:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 112 matching lines @@
     void didChangeTimerAlignmentInterval();
 
     SandboxFlags sandboxFlags() const { return m_sandboxFlags; }
     bool isSandboxed(SandboxFlags mask) const { return m_sandboxFlags & mask; }
     void enforceSandboxFlags(SandboxFlags mask);
 
     PassOwnPtr<LifecycleNotifier<ExecutionContext> > createLifecycleNotifier();
 
     virtual EventTarget* errorEventTarget() = 0;
     virtual EventQueue* eventQueue() const = 0;
+    virtual bool inBeforeUnloadEvent() const { return m_inBeforeUnloadEvent; }
+    virtual void setIsBeforeUnloadEvent(bool inBeforeUnloadEvent) { m_inBeforeUnloadEvent = inBeforeUnloadEvent; }

[kouhei (in TOK), 2014/10/01 11:38:45]

Can you use lifecycle methods in Document instead?

https://code.google.com/p/chromium/codesearch#chromium/src/third_party/WebKit...

 
 protected:
     ExecutionContext();
     virtual ~ExecutionContext();
 
     virtual const KURL& virtualURL() const = 0;
     virtual KURL virtualCompleteURL(const String&) const = 0;
 
     ContextLifecycleNotifier& lifecycleNotifier();
 
@@ skipping 24 matching lines @@
 
     bool m_activeDOMObjectsAreSuspended;
     bool m_activeDOMObjectsAreStopped;
 
     OwnPtr<PublicURLManager> m_publicURLManager;
 
     // The location of this member is important; to make sure contextDestroyed() notification on
     // ExecutionContext's members (notably m_timeouts) is called before they are destructed,
     // m_lifecycleNotifer should be placed *after* such members.
     OwnPtr<ContextLifecycleNotifier> m_lifecycleNotifier;
+    bool m_inBeforeUnloadEvent;
 };
 
 } // namespace blink
 
 #endif // ExecutionContext_h