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

components/sync/base/cancelation_signal.cc

 // Copyright 2013 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/base/cancelation_signal.h"
 
 #include "base/logging.h"
 #include "components/sync/base/cancelation_observer.h"
 
 namespace syncer {
 
-CancelationSignal::CancelationSignal() : signalled_(false), handler_(NULL) {}
+CancelationSignal::CancelationSignal() : signalled_(false), handler_(nullptr) {}
 
 CancelationSignal::~CancelationSignal() {
   DCHECK(!handler_);
 }
 
 bool CancelationSignal::TryRegisterHandler(CancelationObserver* handler) {
   base::AutoLock lock(signal_lock_);
   DCHECK(!handler_);
 
   if (signalled_)
     return false;
 
   handler_ = handler;
   return true;
 }
 
 void CancelationSignal::UnregisterHandler(CancelationObserver* handler) {
   base::AutoLock lock(signal_lock_);
   DCHECK_EQ(handler_, handler);
-  handler_ = NULL;
+  handler_ = nullptr;
 }
 
 bool CancelationSignal::IsSignalled() {
   base::AutoLock lock(signal_lock_);
   return signalled_;
 }
 
 void CancelationSignal::Signal() {
   base::AutoLock lock(signal_lock_);
   DCHECK(!signalled_);
 
   signalled_ = true;
   if (handler_) {
     handler_->OnSignalReceived();
   }
 }
 
 }  // namespace syncer