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

components/sync/syncable/syncable_write_transaction.cc

 // Copyright 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/syncable/syncable_write_transaction.h"
 
 #include <string>
 
 #include "components/sync/syncable/directory.h"
 #include "components/sync/syncable/directory_change_delegate.h"
 #include "components/sync/syncable/mutable_entry.h"
 #include "components/sync/syncable/transaction_observer.h"
 #include "components/sync/syncable/write_transaction_info.h"
 
 namespace syncer {
 namespace syncable {
 
 const int64_t kInvalidTransactionVersion = -1;
 
 WriteTransaction::WriteTransaction(const tracked_objects::Location& location,
                                    WriterTag writer,
                                    Directory* directory)
     : BaseWriteTransaction(location, "WriteTransaction", writer, directory),
-      transaction_version_(NULL) {
+      transaction_version_(nullptr) {
   Lock();
 }
 
 WriteTransaction::WriteTransaction(const tracked_objects::Location& location,
                                    Directory* directory,
                                    int64_t* transaction_version)
     : BaseWriteTransaction(location, "WriteTransaction", SYNCAPI, directory),
       transaction_version_(transaction_version) {
   Lock();
   if (transaction_version_)
@@ skipping 150 matching lines @@
     ENUM_CASE(SYNCAPI);
   }
   NOTREACHED();
   return std::string();
 }
 
 #undef ENUM_CASE
 
 }  // namespace syncable
 }  // namespace syncer