Use C++11 features in core/workers.

Source/core/workers/WorkerConsole.cpp

 /*
  * Copyright (C) 2007 Apple Inc. All rights reserved.
  * Copyright (C) 2013 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.
@@ skipping 44 matching lines @@
         const ScriptCallFrame& lastCaller = callStack->at(0);
         consoleMessage->setURL(lastCaller.sourceURL());
         consoleMessage->setLineNumber(lastCaller.lineNumber());
     }
     m_scope->addConsoleMessage(consoleMessage);
 }
 
 ExecutionContext* WorkerConsole::context()
 {
     if (!m_scope)
-        return 0;
+        return nullptr;
     return m_scope->executionContext();
 }
 
 void WorkerConsole::trace(Visitor* visitor)
 {
     visitor->trace(m_scope);
     ConsoleBase::trace(visitor);
 }
 
 // FIXME: add memory getter
 
 } // namespace blink