Let notifyScriptLoadError() handle already detached ScriptLoaders.

third_party/WebKit/Source/core/dom/ScriptRunner.h

 /*
  * Copyright (C) 2010 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
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 80 matching lines @@
     WebTaskRunner* m_taskRunner;
 
     int m_numberOfInOrderScriptsWithPendingNotification;
 
     bool m_isSuspended;
 #ifndef NDEBUG
     bool m_hasEverBeenSuspended;
 #endif
 
 #if !ENABLE(OILPAN)
+    bool m_isDisposed;
     WeakPtrFactory<ScriptRunner> m_weakPointerFactoryForTasks;
 #endif
 };
 
 } // namespace blink
 
 #endif // ScriptRunner_h