DatabaseBackend should not post a task to an already terminated database thread

Source/modules/webdatabase/DatabaseContext.h

 /*
  * Copyright (C) 2008 Apple Inc. All Rights Reserved.
  * Copyright (C) 2011 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 41 matching lines @@
 
     virtual ~DatabaseContext();
     void trace(Visitor*);
 
     // For life-cycle management (inherited from ActiveDOMObject):
     virtual void contextDestroyed() OVERRIDE;
     virtual void stop() OVERRIDE;
 
     DatabaseContext* backend();
     DatabaseThread* databaseThread();
+    bool databaseThreadAvailable();
 
     void setHasOpenDatabases() { m_hasOpenDatabases = true; }
     // Blocks the caller thread until cleanup tasks are completed.
     void stopDatabases();
 
     bool allowDatabaseAccess() const;
 
     SecurityOrigin* securityOrigin() const;
     bool isContextThread() const;
 
 private:
     explicit DatabaseContext(ExecutionContext*);
 
     RefPtrWillBeMember<DatabaseThread> m_databaseThread;
     bool m_hasOpenDatabases; // This never changes back to false, even after the database thread is closed.
     bool m_hasRequestedTermination;
 };
 
 } // namespace blink
 
 #endif // DatabaseContext_h