Show a warning when using sync xhr.

Source/bindings/core/v8/V8AbstractEventListener.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2009 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:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 106 matching lines @@
         v8::TryCatch tryCatch;
         tryCatch.SetVerbose(true);
 
         // Save the old 'event' property so we can restore it later.
         v8::Local<v8::Value> savedEvent = V8HiddenValue::getHiddenValue(isolate(), scriptState()->context()->Global(), V8HiddenValue::event(isolate()));
         tryCatch.Reset();
 
         // Make the event available in the global object, so LocalDOMWindow can expose it.
         V8HiddenValue::setHiddenValue(isolate(), scriptState()->context()->Global(), V8HiddenValue::event(isolate()), jsEvent);
         tryCatch.Reset();
-
+        if (event->isBeforeUnloadEvent())
+            scriptState()->executionContext()->setIsBeforeUnloadEvent(true);
         returnValue = callListenerFunction(jsEvent, event);
+        if (event->isBeforeUnloadEvent())
+            scriptState()->executionContext()->setIsBeforeUnloadEvent(false);
         if (tryCatch.HasCaught())
             event->target()->uncaughtExceptionInEventHandler();
 
         if (!tryCatch.CanContinue()) { // Result of TerminateExecution().
             if (scriptState()->executionContext()->isWorkerGlobalScope())
                 toWorkerGlobalScope(scriptState()->executionContext())->script()->forbidExecution();
             return;
         }
         tryCatch.Reset();
 
@@ skipping 41 matching lines @@
 {
     return isolate()->InContext() && &world() == &DOMWrapperWorld::current(isolate());
 }
 
 void V8AbstractEventListener::setWeakCallback(const v8::WeakCallbackData<v8::Object, V8AbstractEventListener> &data)
 {
     data.GetParameter()->m_listener.clear();
 }
 
 } // namespace blink