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

components/sync/engine_impl/js_mutation_event_observer.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_impl/js_mutation_event_observer.h"
 
 #include <stddef.h>
 
 #include "base/location.h"
 #include "base/logging.h"
@@ skipping 35 matching lines @@
     ModelType model_type,
     int64_t write_transaction_id,
     const ImmutableChangeRecordList& changes) {
   if (!event_handler_.IsInitialized()) {
     return;
   }
   base::DictionaryValue details;
   details.SetString("modelType", ModelTypeToString(model_type));
   details.SetString("writeTransactionId",
                     base::Int64ToString(write_transaction_id));
-  base::Value* changes_value = NULL;
+  base::Value* changes_value = nullptr;
   const size_t changes_size = changes.Get().size();
   if (changes_size <= kChangeLimit) {
     base::ListValue* changes_list = new base::ListValue();
     for (ChangeRecordList::const_iterator it = changes.Get().begin();
          it != changes.Get().end(); ++it) {
       changes_list->Append(it->ToValue());
     }
     changes_value = changes_list;
   } else {
     changes_value =
@@ skipping 31 matching lines @@
     const std::string& name,
     const JsEventDetails& details) {
   if (!event_handler_.IsInitialized()) {
     NOTREACHED();
     return;
   }
   event_handler_.Call(from_here, &JsEventHandler::HandleJsEvent, name, details);
 }
 
 }  // namespace syncer