[Sync] Replacing NULL with nullptr in code and null in comments for components/sync/

components/sync/engine/model_safe_worker.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "components/sync/engine/model_safe_worker.h"
 
 #include "base/bind.h"
 #include "base/json/json_writer.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "base/values.h"
@@ skipping 106 matching lines @@
     // destroyed (see MessageLoop::~MessageLoop). So syncer will be blocked
     // indefinitely without signaling here.
     work_done_or_stopped_.Signal();
 
     DVLOG(1) << ModelSafeGroupToString(GetModelSafeGroup())
              << " worker stops on destruction of its working thread.";
   }
 
   {
     base::AutoLock l(working_task_runner_lock_);
-    working_task_runner_ = NULL;
+    working_task_runner_ = nullptr;
   }
 
   if (observer_)
     observer_->OnWorkerLoopDestroyed(GetModelSafeGroup());
 }
 
 void ModelSafeWorker::SetWorkingLoopToCurrent() {
   base::Callback<void(ModelSafeGroup)> unregister_done_callback;
 
   {
@@ skipping 51 matching lines @@
       return;
     DCHECK(working_task_runner_->BelongsToCurrentThread());
   }
 
   DCHECK(stopped_);
   base::MessageLoop::current()->RemoveDestructionObserver(this);
   unregister_done_callback.Run(GetModelSafeGroup());
 }
 
 }  // namespace syncer