Show a warning when using sync xhr.

Source/bindings/core/v8/V8AbstractEventListener.cpp

Index: Source/bindings/core/v8/V8AbstractEventListener.cpp
diff --git a/Source/bindings/core/v8/V8AbstractEventListener.cpp b/Source/bindings/core/v8/V8AbstractEventListener.cpp
index 21be4e1c79935cc44ad2dfae3978b61b6b15a619..b4c9d2a3fedc98e12cec72b154e48641da930032 100644
--- a/Source/bindings/core/v8/V8AbstractEventListener.cpp
+++ b/Source/bindings/core/v8/V8AbstractEventListener.cpp
@@ -124,8 +124,11 @@ void V8AbstractEventListener::invokeEventHandler(Event* event, v8::Local<v8::Val
         // 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();