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

components/sync/syncable/base_transaction.h

 // 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.
 
 #ifndef COMPONENTS_SYNC_SYNCABLE_BASE_TRANSACTION_H_
 #define COMPONENTS_SYNC_SYNCABLE_BASE_TRANSACTION_H_
 
 #include "base/macros.h"
 #include "components/sync/base/cryptographer.h"
 #include "components/sync/base/model_type.h"
@@ skipping 18 matching lines @@
 // fail. There is no built-in rollback or undo mechanism.
 class BaseTransaction {
  public:
   // Provide access to the underlying syncable objects from BaseNode.
   virtual syncable::BaseTransaction* GetWrappedTrans() const = 0;
   Cryptographer* GetCryptographer() const;
   ModelTypeSet GetEncryptedTypes() const;
 
   syncable::Directory* GetDirectory() const {
     if (!user_share_) {
-      return NULL;
+      return nullptr;
     } else {
       return user_share_->directory.get();
     }
   }
 
   UserShare* GetUserShare() const { return user_share_; }
 
  protected:
   explicit BaseTransaction(UserShare* share);
   virtual ~BaseTransaction();
 
-  BaseTransaction() : user_share_(NULL) {}
+  BaseTransaction() : user_share_(nullptr) {}
 
  private:
   UserShare* user_share_;
 
   DISALLOW_COPY_AND_ASSIGN(BaseTransaction);
 };
 
 }  // namespace syncer
 
 #endif  // COMPONENTS_SYNC_SYNCABLE_BASE_TRANSACTION_H_