Show a warning when using sync xhr.

Source/core/dom/ExecutionContext.cpp

 /*
  * 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 54 matching lines @@
     String m_sourceURL;
     RefPtrWillBeMember<ScriptCallStack> m_callStack;
 };
 
 ExecutionContext::ExecutionContext()
     : m_sandboxFlags(SandboxNone)
     , m_circularSequentialID(0)
     , m_inDispatchErrorEvent(false)
     , m_activeDOMObjectsAreSuspended(false)
     , m_activeDOMObjectsAreStopped(false)
+    , m_inBeforeUnloadEvent(false)
 {
 }
 
 ExecutionContext::~ExecutionContext()
 {
 }
 
 bool ExecutionContext::hasPendingActivity()
 {
     return lifecycleNotifier().hasPendingActivity();
@@ skipping 182 matching lines @@
 void ExecutionContext::trace(Visitor* visitor)
 {
 #if ENABLE(OILPAN)
     visitor->trace(m_pendingExceptions);
     HeapSupplementable<ExecutionContext>::trace(visitor);
 #endif
     LifecycleContext<ExecutionContext>::trace(visitor);
 }
 
 } // namespace blink