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

components/sync/syncable/read_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/read_transaction.h"
 
 #include "components/sync/syncable/directory.h"
 #include "components/sync/syncable/syncable_read_transaction.h"
 
 namespace syncer {
 
 //////////////////////////////////////////////////////////////////////////
 // ReadTransaction member definitions
 ReadTransaction::ReadTransaction(const tracked_objects::Location& from_here,
                                  UserShare* share)
-    : BaseTransaction(share), transaction_(NULL), close_transaction_(true) {
+    : BaseTransaction(share), transaction_(nullptr), close_transaction_(true) {
   transaction_ =
       new syncable::ReadTransaction(from_here, share->directory.get());
 }
 
 ReadTransaction::ReadTransaction(UserShare* share,
                                  syncable::BaseTransaction* trans)
     : BaseTransaction(share), transaction_(trans), close_transaction_(false) {}
 
 ReadTransaction::~ReadTransaction() {
   if (close_transaction_) {
@@ skipping 20 matching lines @@
                                                AttachmentIdList* ids) const {
   DCHECK(ids);
   transaction_->directory()->GetAttachmentIdsToUpload(transaction_, type, ids);
 }
 
 std::string ReadTransaction::GetStoreBirthday() const {
   return transaction_->directory()->store_birthday();
 }
 
 }  // namespace syncer