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

components/sync/engine/attachments/attachment_store_frontend_unittest.cc

 // Copyright 2014 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/attachments/attachment_store_frontend.h"
 
 #include <utility>
 
 #include "base/bind.h"
 #include "base/callback.h"
@@ skipping 210 matching lines @@
 
   attachment_store_frontend_->ReadMetadata(
       AttachmentStore::SYNC,
       base::Bind(&AttachmentStoreFrontendTest::ReadMetadataDone));
   EXPECT_EQ(read_metadata_call_count_, 0);
   RunMessageLoop();
   EXPECT_EQ(read_metadata_call_count_, 1);
 
   // Releasing referehce to AttachmentStoreFrontend should result in
   // MockAttachmentStore being deleted on backend loop.
-  attachment_store_frontend_ = NULL;
+  attachment_store_frontend_ = nullptr;
   EXPECT_EQ(dtor_call_count_, 0);
   RunMessageLoop();
   EXPECT_EQ(dtor_call_count_, 1);
 }
 
 }  // namespace syncer