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

webrtc/base/signalthread.cc

 /*
  *  Copyright 2004 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 95 matching lines @@
     }
     if (kStopping != state_) {
       // Before signaling that the work is done, make sure that the worker
       // thread actually is done. We got here because DoWork() finished and
       // Run() posted the ST_MSG_WORKER_DONE message. This means the worker
       // thread is about to go away anyway, but sometimes it doesn't actually
       // finish before SignalWorkDone is processed, and for a reusable
       // SignalThread this makes an assert in thread.cc fire.
       //
       // Calling Stop() on the worker ensures that the OS thread that underlies
-      // the worker will finish, and will be set to NULL, enabling us to call
+      // the worker will finish, and will be set to null, enabling us to call
       // Start() again.
       worker_.Stop();
       SignalWorkDone(this);
     }
     if (do_delete) {
       refcount_--;
     }
   }
 }
 
@@ skipping 10 matching lines @@
   {
     EnterExit ee(this);
     if (main_) {
       main_->Post(RTC_FROM_HERE, this, ST_MSG_WORKER_DONE);
     }
   }
 }
 
 void SignalThread::OnMainThreadDestroyed() {
   EnterExit ee(this);
-  main_ = NULL;
+  main_ = nullptr;
 }
 
 bool SignalThread::Worker::IsProcessingMessages() {
   return false;
 }
 
 }  // namespace rtc