Replace NULL with nullptr or null in webrtc/base/.

webrtc/base/asyncinvoker-inl.h

 /*
  *  Copyright 2014 The WebRTC Project Authors. All rights reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 50 matching lines @@
 
  protected:
   NotifyingAsyncClosureBase(AsyncInvoker* invoker,
                             const Location& callback_posted_from,
                             Thread* calling_thread);
   void TriggerCallback();
   void SetCallback(const Callback0<void>& callback) {
     CritScope cs(&crit_);
     callback_ = callback;
   }
-  bool CallbackCanceled() const { return calling_thread_ == NULL; }
+  bool CallbackCanceled() const { return calling_thread_ == nullptr; }
 
  private:
   Location callback_posted_from_;
   Callback0<void> callback_;
   CriticalSection crit_;
   Thread* calling_thread_;
 
   void CancelCallback();
 };
 
@@ skipping 49 matching lines @@
     TriggerCallback();
   }
 
  private:
   FunctorT functor_;
 };
 
 }  // namespace rtc
 
 #endif  // WEBRTC_BASE_ASYNCINVOKER_INL_H_